What are the main signs he is into her?

Guys can make it very complicated for girls to figure out if they like them or not. Girls on the other hand, are pretty obvious when they like someone. Sometimes guys aren't sure though but that's cause they have admitted in the past that they can be blind and slow at figuring it out. Girls over-analyze only cause guys master delivering mixed signals so we kind of have to put the pieces together for the final result. With that being said, what are the ultimate signs that girls should consider before leaving him because he is into her? What are the main flaws of guys that jeopardize having something with that girl he likes?

For example, lets say David likes Monica and wants something with her but he lacks expressing himself (a really common mixed signal) so it confuses Monica of whether he likes her or not and once she's tired of figuring things out, she leaves. David and Monica could have had something but because of David's flaws, she left. Not saying that its his fault, but Monica was having a hard time understanding if he likes her or not. As advice, girls shouldn't take it personal when guys lack expressing affection or emotions. Or how guys like having "space" so it shouldn't be taken personal. There are a lot of mixed signals that guys can deliver and call girls complicated. Then they don't understand why it didn't work out. Well in order for things to work out, guys need to express themselves a little better. They have also admitted that even if they act confident, they're actually really shy when it comes to girls so when they get someone they really like and connect with, the last thing that they would want to happen I'm guessing is for her to leave cause there was more to than him not expressing his feelings to her.

I read something from this Relationship Coach and she said there's so much potential in relationships and a lot of people tend to walk away just cause of the typical: not enough commitment, not enough communication, not enough attention, etc. So its important to know where guys are coming from before its over. Of course, girls have flaws as well but this post is mostly focused on guys. So what are the things girls need to consider before they leave and so the guy won't feel like we just gave up? Or what are the things that guys would like girls to know before they leave?