In comics it is stated that, while in college, he and Mr. Fantastic were roommates, Reed finds out mistakes in the experiment victor is going to perform, he tells victor about it, but they were dismissed...
eventually, the experiment blows up on victors face, he is scarred for life(and was expelled)
but in movies it is shown, that after the change in DNA, he starts exhibiting the symptoms(the still thingy, hair loss), so he starts wearing the mask.
Pardon my English, I want to explain it as elaborate as possible, I think I tried well.