It doesn't. People are assuming that the lack of explanation + his young age = suicide.
It may end up being the case, but at the moment, people are just speculating.
It is true that people his age don't usually die of disease, but that still leaves other options than suicide. The most common for males 15-24, according to http://www.worldlifeexpectancy.com/usa-cause-of-death-by-age..., are traffic accidents, violence, suicide, poisoning "other injuries," and drowning, in that order.
Traffic accident would have been my first guess, being the most common death cause amongst the people I knew. I didn't think of suicide till I read the comments.
CNN Money reports that it was confirmed suicide.
It would say how he died if it were anything other than suicide.
CNN Money says a source close to the company confirmed it was a suicide: