Seamus had passed up on three different chances to tell Dean Thomas how he felt.
The first was at the Quidditch World Cup. Seamus hadn't exactly missed that chance. Ireland had won, and Seamus had quickly lost count of how many butterbeers he'd had during the celebration. Seamus had felt the words on the very tip of his intoxicated tongue when someone screamed. There'd been a lot of fussing just then, and for the first minute or so neither of them could figure out what it was about. Then, Seamus' mum dragged them away, frantically yammering something about Death Eaters. They'd spent half the night in a hot ditch with bugs crawling all over them, and at some point Seamus had decided to tell Dean that if they didn't make it out of this, he wanted him to know that he loved him and had been sweet on him since second-year. Once Seamus had sobered up, he'd immediately set to convincing Dean that it had only been the alcohol talking, and Dean had seemed happy enough to believe him.
The second was when Harry Potter and Ginny Weasley had kissed at the Quidditch party. Dean had been hurt, and Seamus had been there to pick up the pieces, as he had through most of Dean's relationship with Ginny. It had been easy enough, until Dean had started to take it personally. He hadn't been able to figure out where he'd gone wrong, why she'd left him, why she wouldn't just trust him, and in his darkest moment he'd asked Seamus if he was just unlovable. Seamus had managed, just barely, to squeak out a "No," and leave it at that. He'd handed Dean a firewhiskey and sat up with him until sunrise, but he hadn't said a word about his own feelings.
The third was when word first started spreading about Muggleborn Registration. Dean and Seamus had already spoken at length about the positions they'd be in if Voldemort returned to power, and Seamus knew immediately that Dean was no longer safe at Hogwarts, or anywhere else where he could be easily found. The day he received a letter from Dean expressing the same thoughts, he'd collapsed onto his bed and cried for hours. He hadn't cried in years, but once he'd started he couldn't stop, and every thought of actually having to reply to that letter, to tell Dean to leave and not say a word about where he was going, made the tears worse. Seamus had almost told Dean in that final letter, but he'd held back. There was no sense in giving Dean one more thing to worry about.
Seamus had passed up on three different chances to tell Dean Thomas how he felt, and as he walked past the bodies strewn across the Great Hall after what would later be called the Battle of Hogwarts, there was only one person that he desperately wanted to see alive, and only one thing he desperately wanted to say.