1.they have a history.
2.you can see it in both neal's eyes and alex's eyes how much they mean to each other.
3.the flor paper(it's their thing)
4.she know that she can trust him(unlike sara).
5.she knows him better.
6.he knows her better.
7."you saved me again"neal to alex (2x16)
8."you saved me"alex to neal(2x16)
9.they get each other better,they understand each other better.
10.they way they look at each other
11.it's obvious that she's in amor with him.
12.it's obvious that he has feelings for her.
13.the hug in s2 finale.
14.the kiss in s2 finale.
15.with sara it's one sided and you can clearly see...