lots and lots of EPO shots.
anyway, when i was around 20 (a decade ago) and racing mountain bikes was my life, it seemed like becoming a pro at that point was something that was possible with a lot of dedication, work, and talent. what made it seem undesireable was that so many of the pro riders i'd meet at races seemed to be total bums. flat broke, sleeping in the backs of their beat-up old cars and eating beans from a can.
lots of these guys were talented and fast riders, but it was clear that unless you were in the top half percent (or less) of professionals, there was not a living in pro racing.
the stakes and standards in road racing are much higher than mountain (no one smokes weed after races, for instance), and the chances of success are far less.
when i transitioned over to road riding, i raced for two years and went as far as cat 3. i considered (and still do) myself a very strong rider, but i really got my ass handed to me when i hit cat 3. it became clear that without an amazing amount of training, coaching, nutrition, discipline, and raw talent, going any further was impossible.