I love you was so hard to say that asking Bilbo to stay with him forever was easy.
At first, the difficulty lay in trying to determine what Bilbo Baggins was. Everything he knew about hobbits before the quest led him to believe Bilbo would be timid, a burden beyond anything that Thorin wouldn't even pretend to try to waste his energy saving because surely Gandalf was wrong about their burglar's necessity, that's what Nori was for. At least the hobbit did seem to value that dwarrow deserved their home just as much as any other race. This immediately put Bilbo above the countless others that denied Thorin and his ilk the same rights that everyone else had in the comfort of their homelands.
But as much as Thorin had made false assumptions about hobbits due to their relatively isolated natures, Bilbo himself was no better at providing accurate information. He confirmed that hobbits were timid things who had no desire to fight. But Thorin had seen Bilbo attack an orc five times his size who he knew had killed countless others. Bilbo's tameness was undermined by the hobbit's sharp tongue whenever he really wanted to prove a point or felt comfortable enough around the company to deliver a dry yet piercing joke that almost made Thorin laugh (the dwarf king would later find out how clever Bilbo's tongue really was, for both defense against a dragon and other more.. suggestive activities).
Bilbo also claimed that hobbits valued good food and good company above all else, but that didn't seem true at all. Even from their night at Bag End Bilbo defended his property most ardently. He was furious that his things were damaged and would often complain about greedy relatives trying to get their hands on his silver. If Bilbo were as abstemious as he claimed he would not hoard his things so nor value them as much. He was a single hobbit who lived well above his means, for Thorin that meant that perhaps Bilbo would not be as put out with luxury as he claimed.
So what's a dwarrow to do when the hobbit he was growing impossibly attached to who was nothing but a creature of contradictions? A hobbit who shivered violently at night unless wrapped up in furs or Thorin, but walked barefoot across the Misty Mountains? Or who claimed to know nothing of the dangers of the world but could talk trolls out of a meal?
At first, these contradictions infuriated Thorin and in his confusion he took out a lot of his frustrations on the hobbit himself. The dwarf only grew more frustrated each time a stabbing guilt pierced him when Bilbo was put in danger and it was all Thorin's fault. After the stone giants Thorin had legitimately thought it best that Bilbo leave in order to sever the growing connection felt by Thorin in order to allow it to die a natural death. Even this thought was complicated as Thorin blustered a lot after he thought Bilbo had actually left (or worse, died) after the goblin caves, but his immense relief when the hobbit turned was unparalled. He knew his external actions made it seem to Bilbo that Thorin hated him, but Thorin believed that it was a better alternative to telling the hobbit just how much Thorin really cared.
This was mostly due to the fact that Thorin was positive that there was no way Bilbo could care for him in that was. How could Bilbo possibly feel the same depths as the dwarf? Thorin was becoming increasingly sure that the supposedly gentle creature was his One, which only brought more torment since he was fairly confident hobbits with their fertile people and many children did not have an equivalent concept.
Thorin felt like his love was choking him, he realized this even as they ran from the wargs. His happiness that Bilbo hadn't returned home was made brief by the continuing danger that surrounded their company. The pale orc was coming after him and he had no idea what to do but stand and fight.
Thorin was sure that he was going to die, not even halfway to their goal. The only thing he could think to do was make sure that he would be the first to die, to try and take down the albino menace that haunted him so that the others(Bilbo) had a chance. The dwarf fought the same as he did the first time he came across the orc, with the same passion and determination, but he made a disappointingly short resistance before everything went black. The only images after that point were brief glimpses of his beautiful hobbit against the desolate fiery background.
When Thorin finally came to, long after the battle was over, his first instinct was to be angry again.
"What do you think you were doing?" He knew his voice sounded harsh, didn't need Bilbo's flinch to tell him that. "You nearly got yourself killed." Because that was Thorin's biggest issue, because he would rather not wake up at all then wake up in a world without Bilbo Baggins.
But then… then he realized something. Thorin really couldn't see himself living without the hobbit and in that moment something in his chest shifted, lightening the immeasurable weight that he felt stacked against his chest.
Maybe it was the smoke inhalation or the adrenaline of everyone surviving, but when he stepped forward with a "and I've never been so wrong in my life," and embraced his hobbit, everything felt right in the world for the first time in longer than he'd like to admit.
Thorin held onto to this feeling and although lack of privacy and pesky injuries made Thorin pass out every so often, he finally found the opportunity to reveal to Bilbo what he wanted. He knew he had to do it, even if Bilbo didn't reciprocate. Thorin's feelings of frustration about what he felt had altered themselves into the comfort that only finding one's One could bring. For this reason he knew he had to say something. If Bilbo could brave more fears than he could have ever possibly known in the Shire, Thorin could not be a coward about his love.
At some point when the rest of the company was on the lookout for scouting wargs or fetching supplies, Bilbo and Thorin were alone at their campsite. Bilbo reached down to remove Thorin's bandages to replace them, but as soon the hobbit's hand touched the bloodstained cloth the dwarf halted him. By placing his hand over Bilbo's Thorin guided it to over his heart. Bilbo stilled, looking questioningly at his leader but with body language betrayed an earned trust that Thorin hoped he would not lose.
Thorin took a deep breath, not doubting the hobbit could feel his pulse rabbiting within his chest, before speaking slowly and lowly. Not wanting anyone else to hear his confession but also wanting to convey his complete sincerity.
"What I am about to say may seem like it doesn't make sense and confuse you, master burglar." He said, his chest vibrating with the bass of his voice as he looked Bilbo in the eyes, "but I must tell you something as it is not in a dwarrow's nature to not at least try to capture something so precious." Bilbo's breathing hitched and his hand pressed more firmly above Thorin heart which literally beat only for the hobbit. They stared at each other for another long moment as Thorin committed to memory this last moment of uncertainty. After he spoke his next words he would know exactly if Bilbo had ever even considered the possibility of the two of them together; Thorin found himself savouring this last point of limitless possibilities before saying as clearly as he could:
"Bilbo Baggins I am in love with you."