Richard's comment on this question made me realize that Doc Brown seems to be a bit hypocritical. Allow me to explain.
The whole reason Marty ends up in 1955 (as opposed to any other point in time) is that the 1985 Doc set the controls to that year, because he wanted to visit himself on the night he invented the flux capacitor. Unfortunately, the Libyans show up and gun him down, Marty tries to escape in the DeLorean, accidentally goes back in time, and you know the rest.
Marty finds the Doc in 1955 and asks for his help. He eventually tries to warn the Doc about the Libyans, but Doc refuses to listen, and repeatedly lectures Marty on the importance of not changing the past.
And therein lies the problem. 1955 Doc knows that you shouldn't change the past, but 1985 Doc doesn't. At some point over those 30 years, he managed to forget the whole "time travel paradox" issue, to such an extent that he was going to visit himself 30 years earlier (to be fair, this is sort of explained in the movie itself, albeit in a rather unsatisfactory manner- when Marty gets back to 1985 and sees that the Doc has survived the shooting, he asks how the Doc knew about the Libyans; Doc reveals that he read Marty's letter, and adds "I figured, 'What the hell'").
Is there some explanation for why the 1985 Doc thought it was okay to visit himself, but the 1955 Doc was dead-set against letting Marty tell him about the Libyans? Do we know why the 1985 Doc was going back in time in the first place (apart from the fun of visiting himself)?