Well, it's native to North America, where the lack of written history (and later the killing off of a massive percentage of the population put a serious dent in oral history traditions) so we'll never know for sure, but presumably at some point someone noticed that the smoke from "them-thar" burning leaves had a curious effect on our nervous systems...
Native Americans "used" tobacco, but at least originally, and even later mostly, as a mood/consciousness altering substance for ceremonial purposes (and as far as anyone has been able to tell, humans as a species have been doing that with one thing or the other, as and when available, since our remotest ancestors first fell out of the trees...). It was "decadent" Europeans (at first, and then everyone else) who developed the habit of regularly smoking it, and even that, as a widespread phenomenon, came pretty late in the history of its usage (after snuff, and occasional smoking by a relatively few rich people). And by late, I mean the mid-late 19th and 20th centuries.
As to that, I doubt we'll ever really understand the real "explanation" for it (despite lots of finger-pointing and politically-motivated blame-throwing.) Presumably it was/is the confluence of any number of social, cultural and scientific reasons overlaying an inherent genetic predisposition - during more or less the same time period, consumption of drugs of all sorts mushroomed to never-before-seen proportions ... Among other things, they became available outside their native geographic ranges where traditional cultural factors had kept their use in check, partly because "science" and technology - in several guises - resulted in much stronger, more effective, and also more physically addictive substances than we'd had to deal with previously and had the (extremely long) time to adapt to via natural selection...
