Because it had to be done.
He is in love. He's a man, so he doesn't have any problems admitting it. Well, he can admit it to himself, anyway, and maybe some day he'll find the guts to tell her. He'll never tell his little brother but then, what's so manly about telling your little brother that sort of thing? It isn't that he's scared to tell him, so much that, well, when you really come right down to it, you couldn't explain a thing like that to Luffy.
Something will have to be said sooner or later, though, because there is no way in hell he is letting this one go. That girl is a slammin' hottie, with smoldering eyes that call to him and flaming hair that he longs to touch and a fiery temper that makes him morbidly curious as to what she'll be like on a more, ah, personal level. She burns so brightly, with her light and energy and passion, and he finds himself drawn to her fire.
She's wasted on those goofs in his little brother's crew, that's for damn sure. They're good kids, no question, but that woman needs a real man. A real man who will treasure her and her strength, who'll take care of her and who won't put up with her bullshit any more than she'll put up with his… a man, in short, like Ace.
Here is a girl who could come with him anywhere in the world, who could probably take him places he's never dreamed of, a girl of whom he would never tire and who will have the strength to stay with him forever.
They'd spend a lot of time in hot countries. Ace likes hot things: spicy foods, hot climates, smoking hot women. Alabasta, for instance, would be a perfect place to spend some time. That's where they met, after all. She had worn a dress designed to set men's hearts - among other things - on fire. And damn if it hadn't done it's job. Ace likes that dress. A lot. When she wore it, she was gorgeous and perfect and beautiful, and then some loser would stare at her too long, and that fiery wrath would descend on the unfortunate's head. He likes that even more.
If he's honest with himself - which he tried to be - Ace knows he isn't the best prospect for most girls. He's good looking - blazing hot, he'd like to think, although he doesn't really believe it - and strong, and if he ever put his mind to it he could make enough money for them to settle down comfortably, if such a thought ever occurred to him. That said, he's a pirate, and not even a captain, at that. Sure, he's the commander of over half the Whitebeard pirates - which is awesome - but he's also a narcoleptic pyromaniac who comes with some unfortunate baggage. Because - and this is the part he struggles with - not for any woman in the world will he give up his little brother, idiot though he is.
Maybe that was what attracted him to her in the first place. She already knows his emotional baggage, which he used to think would haunt any relationship he ended up in, and she seems to really like the idiot. No, she doesn't just like him; she would follow the kid to the end of the world. Ace hopes that someday that devotion will be transferred to him, but he knows he'll be happy if she cares about him even half as much as she cares about the baggage - as long as she shows it in a different way. He's not a stretchy little rubber monkey, after all - he's a man of fire, and she's just going to have to deal with that, because whether she likes it or not, he's going to be around for a long time.
Yeah, he'll tell her some time. A fiery woman like that is all a fire man like him could ever hope for. But he won't tell Luffy. It's enough that she knows about the baggage; the baggage doesn't need to know about her. He wouldn't understand, anyway.