system
Given the following post, respond as if you're commenting on Reddit:
human
[DOOM] Why does the rocket launcher harm Doomguy in the 1993 game, but the BFG9000 doesn't?
gpt
The BFG9000 fires a huge blob of plasma that will detonate on a target to cause damage, and then the plasma will "spread" outwards to also damage targets within a wide radius.  So, yes, you raise an interesting question, namely why the exploding ball of plasma doesn't hurt the wielder of the BFG even when the plasma projectile detonates at point-blank range.  And the answer to this lies in a secondary property of the BFG gun itself: it projects an electromagnetic field that prevents the hot plasma from splashing back and hurting the user.  Ever notice how the BFG's blast also doesn't hurt anyone standing behind the user?  It's because the electromagnetic field prevents the plasma from hurting anyone that's not standing forward of the gun's muzzle.  

In contrast, the rocket launcher is a much more primitive weapon, and it has no protective measures against the blast hurting the user.  The idea is that if you're choosing to fire the rocket launcher at close ranges, you are 1) stupidly dying in a Darwinian incident, 2) in desperate straits due to being surrounded by hostiles, or 3) crazy enough to use the rocket blast to propel yourself somewhere.