There are other sports that are similar to freediving in sense of solo practice, I'm talking about rock climbing here. Soloing (no rope, no partner) in rock climbing is at least as dangerous as diving solo, any mistake results in certain death. However, I know a lot of people who climb solo and I do that myself. I believe there are two critical moments common to all potentially dangerous sports when they are practiced solo:
1) Make sure you know what you're doing.
2) Make sure that you want it.
With 1 and 2 in place - just go for it. Still, diving or climbing with a buddy is more fun!
Solo one has to be very coservative to be safe so less fun but still fun.A short dive can be fun too.