Hohenheim is no stranger to the pain of loss, but he does not experience the overwhelming beauty of joy until he meets Trisha. When she stays with him after he reveals his past, he finally understands what love is, and he cannot imagine a more wondrous feeling.
He marries her, because he wants to proclaim this feeling to the world, to shout, look at this beautiful woman beside me, look at this beautiful woman who knows everything and still wants to be my side for the rest of her life.
Hohenheim does not expect for Trisha to become pregnant, had not even thought something like this could happen to him, someone who has been a part of only destruction, never creation. It must be a miracle, he thinks, and for the first time truly considers a world beyond even alchemy.
He ends up with two sons, both golden-haired and golden-eyed like him, and every time he looks at them he feels a dizzying rush of pride and happiness. They are marks of his existence, proof of the fierce love between himself and Trisha, and Hohenheim understands what it means to be proud of himself.
His family becomes the centre of his world, and Hohenheim is determined not to lose them to time. So when he discovers what Homunculus must be planning, he has only one course of action. To protect Trisha and his boys, he must leave, and Hohenheim experiences what it means to leave someone for their own good.
It hurts, even though he knows, logically, that it's for the best.
When it's all over, and Hohenheim is kneeling in front of Trisha's grave, he realizes that he didn't really live until he met her. He didn't understand what it meant to be human until he met her, because he did not care enough for anyone until then.
And though he doesn't quite want to die yet, because he wants to watch his boys grow into men, Hohenheim dies with no regrets. Being human, even if he was barely one, was fulfilling, in the end.