Most Helpful Guy
It's plain English, I'm afraid. Those guys literally aren't ready to commit to one person, in any real way. Translation: I like you, but I'm not your husband. Gimme the booty, don't gimme the responsibilities.
Ignore them, unless you are also just looking for fun. If you want to get serious, I'm afraid you will typically have to go out with a much older man than the 18-24 crowd. Not always, just most of the time.
Most Helpful Girl
Maybe they just aren't where they would like to be at that point in life to want to settle down just yet. Either they want to have more job or financial stability, or want to meet other girls without getting too serious.