As Superman, the world depends on him. He knows he shouldn't have too much else on his plate. If he was to have a relationship, a life, a family, he knows it's something he'd have to constantly abandon. Something he'd have to keep secret. But he just couldn't ignore his feelings for you.
You've been on your share of dates, mostly at each others homes for privacy. He's over frequently, and yet he's never put a label on things. For a while, you didn't ask. It didn't matter much to you. But now it's been months and you can't handle not knowing. So, you ask.
He sighs and shifts nervously. "It's not that I don't want to be with you. You're beautiful, smart, funny, talented. I mean, I've never felt this way," He starts, rubbing his head. "But you know that I can't really be with you. Or anyone. I'm at the planet's beck and call. I never know when I'll go or when or if I'll come back. I can't do that to you."