Hi Michael -

The only reason the photos look the same is because they're in black and white. The color photos show the difference between asteroids and comets more clearly. Aside from that the comets and most asteroids are composed of different stuff that haas undergone different changes -

"I just don't see one round-ish comet/asteroid gravitationally attracting another oe and somehow through basically no forces other than sitting next to each other they'll merge into another larger somewhat spherical object?"

Basically, if either comets or asteroids are sitting next to each other, their gravitational forces will attract them to eadh other. After the meet up, they merge together for a long time - billlons and billions of years. Note that that is "either -or" - while some asteroids are burned out comets, that is only a few of them - asteroids and comets are for the most part different, but there are signigcant exceptions

"loose molecules"- I should better have said "loose atoms" but these combine into "loose molecules"