Why Does Every Girl That’s Interested in Him End Up Just Wanting to be His Friend?

Via Reddit comes a question I can really identify with:

Do I have Nice Guy Syndrome?

I’m male, 25.

This keeps happening: I meet a girl, there’s flirting going both ways. My friends all affirm the girl is flirting with me. (One of those friends is a girl for what that’s worth.) Me and the girl start texting routinely. She initiates every conversation. I get excited, because I like her, and my friends say she likes me, and they seem right. Over the course of a few days, the girl opens up (only through text though) and tells me about her personal problems. I answer her questions in the nicest way I know how. Then in a particularly flirty texting conversation (again going both ways) I tell her I have feelings for her. Then in a wishy-washy way, they get the message across that they don’t feel the same way. Then they continue to tell me about their life problems, and they often seem to feel very sorry for themselves, and it seems like they just want my validation. It gets excessive, and I think they know that too, because they constantly apologize for how often they come to me with their problems. I’m not really sure what it’s about. But when they tell me they don’t feel the same way, I kindly drop the subject, no passive aggressive talk, absolutely no guilt-tripping. And the girls always tell me that I’m a really nice guy, and I’m left scratching my head as to what all the flirting meant, and my friends don’t get it either. Anyways, I don’t mean to sound angry or anything at these girls. The problems they mention show that they’ve had hard lives, and I could see how texting someone who helps them feel better is something they wanna keep doing. I just don’t get why they flirt with me so hard at first and then say ‘no’ and then keep wanting to vent all their problems to me… me of all people. They barely know me. (This happens A LOT.)

How do I break out of this cycle? And do I have Nice Guy Syndrome?