What I like about hiking alone:
Solitude: time to think, ponder, pray
Quiet: feeling closer to nature, seeing more wildlife
Freedom: to go my own speed, go as long as I want, stop when I want, take a side trip if I want, stay an extra night if I want, etc.
Physical challenge: pushing myself without having to push someone else or hold them back
Mental challenge: going it alone, it's all on me--route finding, maintaining pace, staying motivated, being safe, not getting lost, etc.
Nobody to laugh: when I read the map wrong, take a wrong turn, miss a switchback, take 10 throws for my bear line, etc.
I like hiking with others, too.