Angels have one and only one mate throughout eternity. This is a very long time. However, the mate may not be an angel. Some angels have demon mates, although this has only been known to happen once in recorded history, and that isn't human recorded history angels have recorded history a lot longer than humans have, others take monsters, these are often poor relationships if with werewolves as they do not remember most of the more passionate times. Even still angels take angels as mates. If an angel is mated to another angel then the relationship would have been kindled as fledglings so they knew no different. When angels are not mated to angels it can take centuries for them to find their mate, and even then it can be strenuous for their mate and them alike.
The Angel of Thursday was beginning to believe that he did not have a mate. Until, that is he was sent on a perilous mission to save the righteous man from the depths of hell.
Oft times humans believe in soulmates and love at first sight. This belief stems from those few humans who are destined to be angel-mates. It is very rare for these gifted few to already know of the supernatural world when they meet their angel. Even rarer for them to try and kill said angel. And rarer still for two such people to be related so closely as this pair.
The righteous man knew his mother had believed in angels, but the righteous man just couldn't believe that good existed in this world of literal demons. That is until he stabbed his angel with a knife that could kill anything.
Castiel had been told he would know the righteous man when he saw him. His soul would be the brightest soul in hell. What he didn't expect was for there to be so many bright souls in hell. Granted most were dimmed and blackened by years of torment. Most souls could only withstand a maximum of 10 years before giving in and taking up demonhood. When Castiel entered hell, a month after the righteous man, he had already suffered this long.
Castiel spent 30 years looking for the righteous man. His wings were sore and if they had not already been black they would be so by now. 20 years into his search a cheer rang through hell. That was when Castiel knew the first seal was gone. The righteous man had drawn blood in hell. He had failed his first mission so he was to complete his second. To raise the righteous man as the only one who could stop the apocalypse.
10 years after hells cries of success, heaven rejoiced as a single message ran through the host, "Dean Winchester has been saved." A message also ran through Castiel's body. A bond formed in the moment Cas touched Dean, and this bond would neither weaken nor strengthen for a long time.
In fact the first time the bond strengthens was not that long of a time from when Cas lifted Dean from Hell. It was the first physical meeting of Angel and Human. It was as the blade of the Demon Killing Knife was sheathed in Cas's chest. It was the exact moment Dean's eyes widened in disbelief. Cas felt the shift in the bond as it became more profound.
The second time it shifted was the first time Cas died. He died to protect Dean, and as such the bond strengthened. This time Dean was able to feel the shift. He was unsure as to what it was though, so pushed it to the back of his mind to be ignored at a later date.
Whilst the pair where in Purgatory the bond continuously strengthened. Dean was beginning to realise what he was ignoring but still had no idea. Castiel felt the bond strengthening as Dean hunted for him. Granted he didn't know Dean was hunting for him, if he did he would have stopped him. Not because of the bond but because of the Leviathan. It was dangerous for Dean to be anywhere near him, Castiel knew this which is why he left. Dean didn't, which is why he hunted for the angel.
Then when Cas returned from Purgatory, Naomi was trying to break the bond. She spent weeks getting Cas to kill a falsified Dean over and over again. She made him do this until there was no hesitation, no noticeable flinch as Cas killed the fake Dean. But once he was told to kill the real Dean, once he could feel the bond between him and the Dean he had been told to kill, he couldn't do it. He could injure him, he could break his bones, but he would not could not kill Dean.
Naomi got many things wrong with her fake Deans. One of the things she got wrong was that Dean would never beg Cas for mercy, he would try and get the angel to come to his senses. This, in the end, is what freed Castiel of Naomi's control.
The bond reached its strongest yet the last time Dean died. Cas felt the bond flicker as he waited in Metatron's office in heaven. When the other angel arrived Cas could not feel the bond for the first time in 6 years. This worried the angel as even when Dean had died before Cas could still feel their bond. "You draped yourself in heaven's flag but ultimately it was all about saving 1 human. But guess what! He's dead too!" No, that couldn't be true! Dean couldn't be dead! Cas couldn't live with that!