He Doesn't Commit to the Near Future
Just because a guy is with you does not necessarily mean he sees or wants a future with you. Most guys just want to get laid, so there are guys who will get into a relationship with a girl simply to have it on a regular basis. They get with you in order to always have access to you. If a guy really wants a future with you and not just to have sex with you, after a couple months of dating, he'll be including you in near future events. Buying tickets to a concert next month or inviting you to a friend's birthday party are his ways of telling you he wants to keep you around. You can test the waters and ask him if he wants to go with you to upcoming events. If he can’t commit to something a month or so out, he's probably not thinking this is going to last. Or, he doesn't want to go public and make it more than it is.
And after a year or so of being together, there should be talks of moving in together, kids together, marriage, something that puts you two together 5+ years from now. If he never brings it up or dismisses it every time you do, then he's likely just with you for convenience and will leave you once someone "better" comes along.