In Beauty and the Beast, Belle fell in love with Beast who was very rude and mean. Gaston was mean to, so why didn't she have to heart to fall in love with him as she did with Beast.

I think the difference is once she got to know him she saw in Beast potential for change, hidden goodness and he had the trauma of his transformation and isolation to go some way to explain the badness he hid his better side beneath. Gaston had no excuse for his behaviour we're aware of, never gave Belle the impression there was anything in him hidden or worth bringing out, and even when proposing to her showed no interest in anything about her but her appearance (I mean, Beast liked her looks too, but he noticed and cared for her personality as well by the time they were getting romantic). There was something in Beast underneath his bad side, which I don't think was true of Gaston.
I think with Gaston she was aware there was no hidden depth or compatibility, and aware he didn't and never would love anything about her personality.

I think that it is because Belle pitied the Beast, so she "helped" him, whereas Gaston was a perverted, mean and rude man.

Because Gaston never thought of locking Belle up in HIS house. jk Heh. I agree with the above comments.

cuz she was beasts prisoner in the beginning cuz she didnt want her father in their cuz he was sick so while she was there she got 2 know beast she already knew gaston n hated him i dont blame her i do 2 but beast just had a bad temper n she helped him plus he saved her

Because Gaston wasn't in love with Belle he loved himself! He just wanted to marry her because she was beautiful. Beast was actually nice the whole time, just misunderstood.