The school of thought known as existentialism takes up this question directly. For an existentialist, nothing has meaning intrinsically. Life and behavior both are given meaning by culture, language, society, history, etc. The meaning of life then is, for better or worse, what we say it is.
It was Shakespeare who once wrote this line, which nicely sums up this view:
...for there is nothing either good or
bad, but thinking makes it so.
I don't see what difference it would make if we were to assume hypothetically that life does not have any meaning at all. Most of us would still go on living the same lives we have always lived and doing the same things we have always done. In fact, it might be a relief to have the question settled in our own minds, so that we could forget about it and go on about our business. You can't ever find out the answer to the question of whether life has meaning by asking other people. Some of them will give you very positive and reassuring answers--but they don't know any more about it than you do.
This is one of the biggest philosophical questions around, and there is no way to answer it conclusively. Many people do believe, for various reasons, that their lives have meaning. Please follow the links for exhaustive discussions of views on the meaning of life.
Generally, there are a couple of types of reasons given to say that life has meaning. The first type of reason is God-centered. These arguments say that supernatural powers have a plan for how the universe should work. Our lives have meaning if and when we try our best to understand and carry out this plan.
A second type of reason is based not on God but on our own minds. Thinkers who propose this idea argue that our lives our meaningful because we believe they are. If you think your life has a meaning, then how can you be wrong?
Of course, these people could be wrong. Life may have no meaning. But there is no way to prove it either way.