Everything has a reason, or reasons for happening or existing.

There are causes for meaningless things, which implies that those meaningless things are not as meaningless as we claim they are. 

I suppose, that the reasoning for life is to create something to leave behind, leave your ideas and beliefs with others. Create new things, build, and love. Destroy, smash, break, burn and reforge it all. Define meaning to the meaningless, inform others of your ideas. Humans thought is also what is evolving. Humans are mighty in mind, even if they do not act like it.