All Seamus wanted was what other boys and girls had: a boyfriend. But not any boyfriend— he wanted Dean.
However, what he had was a best friend that spent a year talking non-stop about his girlfriend, Ginny Weasley. Seamus liked Ginny, but that year he could have punched her.
He thought his opportunity had come when they broke up, but then Dean found himself a boyfriend. Ah how much that had hurt! What did Justin have that Seamus didn’t? If Dean wanted a boyfriend why look for him in Hufflepuff and not in his best friend, the person who knew him best, and that he knew was gay?
So he spent the next few months hearing Dean sing the praises of Justin. And because Dean knew he was gay, he was prompted to give his opinion about Justin’s looks. He wanted to punch Dean.
Then the war came, crueler than ever. The Ministry fell under You-Know-Who’s power and Muggle-borns were banned from Hogwarts. That included Dean.
He fought the Carrows and tried to protect the other students as best as he could, all while wondering where Dean was, if he was even still alive. At night he imagined a hundred scenarios in which they would meet again; sometimes they kissed and lived happily ever after, away from the horror of the war, others he found Dean when it was too late.
The day of the last battle came, although none of them knew it at the moment, and with it came Dean. Alive and well. Seamus almost kissed him senseless in front of all the DA members. Instead, he contented himself with a tight hug.
They fought, and he promised himself that if they survived he’d confess his feelings. But Dean thwarted him again. At Shell Cottage he had gotten the chance to know Luna Lovegood as never before. They were now dating. Seamus wanted to kill Dean. Instead, he congratulated him and swallowed his feelings once more.
It was July when Dean and Luna broke up. Seamus waited for his friend to heal, and then in August (because he wasn’t patient enough), he told him. Dean was surprised at first, but he agreed to date just to see how it went. Seamus would have shagged him then and there.
A year later they were still together. And ten years later, Dean was on one knee asking Seamus to marry him—it only took him half a second to shout YES.