I almost always solo.
The only times I play with someone else are when:
1. A new player needs help with missions or the arcade
2. I'm farming RT - I can never kill the boss on my own
So, why shouldn't I play solo? Besides the two things listed above, why shouldn't I play with a party?
Why not to solo:
1. Honestly I hate soloing, because it's lonely
2. 100% of the time I am at 1 bar with lag (I fell in love with SK before I realized. I can't stop now)
3. I heard a rumor about enemies dropping more loot and being more difficult. (Could totally be false though, don't hurt me)
4. More people attacking, faster runs for crowns if everyone is skilled to a certain degree
5. Guild runs are like, the best thing invented. I mean everyone knows each other and what weapons you have and how terrible you are, so you don't have to explain yourself all day to strangers.
6. C'mon it's fun to watch newbies struggle a little bit. "What does this key do?" *stands around for 10 minutes trying to figure it out* *Vanguard dies of laughter*
7. Parties aren't called parties for nothing.
Why to solo and not play in a party:
1. Faster speed runs because you only have to wait for yourself
2. You won't be embarrassed if you die a lot (That chronic 1 bar death though)
3. People who solo tend to be amazing (Well...)
Parties for life. Also this list is more geared towards me and my internet.