I have known a guy since I was 15. We tried to date before, but it didn't work out. I was upset recently and he saw that I was and began talking to me again. We are 24. He sends me the sweetest messages. We were talking alot, but then all of a sudden he just stopped again. What is going on?

That's hard to say. Guys do a lot of weird things and we just don't know why. He could probably be busy or he might be involved with someone else. He could even have some internal conflict within him that we don' know about. It's all up in the air.

The best way to know is by asking him. I'm completely aware that it's really awkward to just straight up ask someone this, but just don't think of it that way. Just think of it as finding answers.

But from what I see, I feel like he's just wasting your time. If he's really on and off, he isn't worth your attention. Everyone deserves someone who is committed and knows what he/she wants. You can still be friends with him, but I wouldn't expect much from him since he seems to have a tendency to not know what he wants.

Maybe he is afraid that it's not gonna work between you two again and he doesn't want to get hurt...

I was in the exact same situation, except I was the guy. There was this girl I met in elementary school and we are still friends today. I tried to ask her to be my girlfriend on Valentines day but she declined. I asked her why and she said she didn't want to ruin our friendship. I stopped talking for a while because I was kind of embarrassed. We then talked for a while and we became better friends than we ever were. I am not really talking to her right now and I personally do not know why. What I think I am trying to say is that friends grow apart sometimes. Mostly after asking out that person and they said no.

Many possibilities.

One no one mentioned is maybe he is a nice sweet guy that saw a friend hurting and chose to be there but has other things in his life including a possible significant other.  Seeing you were back and not needing help anymore, he returned to his life.

He may have never meant for his friendship to become a more intimate relationship or as suggested by others he questions the rightness of it.

Ask him to meet you at a nice neutral location, possibly a restaurant near his work and close to the end of his work day.  This will not appear to be a possible date as no one goes directly from work to a date, you always at least clean up. Have a relaxing dinner and ask if there is a problem.  Let him know you are not trying to force an issue but you would like to know if he is interested in a relationship, because you are, but you are willing to just be friends and look elsewhere if he is not. 

He might be interested but have perfectly valid reasons for not pursuing it or appear to be pursuing it at the moment.  Emergencies, including work emergencies, happen and if your relationship had been casual and no planned time together were interrupted, he just may have not considered calling.  That is not rude or ignoring you.  Consider, do you call all your friends and/or family when something comes up?  No, you only call those you are expected to meet and or help.  That does not mean you do not care about the others or indeed love the others.  Men are like that, only moreso, they only start really telling what is going on that might interfere with shared time if they are already in a relationship.