Reply maybe?

Why are some people sending messages and then not even bothering to reply after we take the effort to reply to them? I don't understand it. Is it because we send back short replies so you aren't interested or wee seem uninterested? We are only replying with the same amount of effort you put it. If we get a two line message, we will return a 2 line message. If we get a 5 paragraph message, we will put in the same effort. So why do you send us messages, we reply and then you don't again? Why bother sending one in the first place? And why add us as friends?