For me isolating when I was depressed from others I had previously enjoyed a safe relationship with is a very bad thing, it fed the spiral downwards, into a very unsafe place. Withdrawing enables me to justify why life isn't worth living, because I no longer felt connected to anyone or anything.
By nature I am a very reserved person, and don't feel the need to be part of a large social network, and being in crowds, just triggers me due to the noise and feeling unsafe. Being comfortable with alone time and socializing with a few safe people is very different to isolating.
Isolating is when I wish everyone would just leave me alone, because I can't stand feeling or talking or thinking because I am so consumed with loathing, sadness and self absorbed in my own misery.