"I tour alone because I don't like people asking me why I tour alone."
Well, if you didn't tour alone they likely wouldn't ask, would they?
For me it's the complete disconnection from my usually hectic life, the amazing slow-down of time, and total peace! I love to stop and smell the roses/take pictures/breathe the salty sea breeze, etc, etc. When I tour it doesn't matter if I take a rest day today or tomorrow. All the distractions are eliminated and life becomes really simple.
I love meeting random other cyclists at night at the campgrounds. And it's a really nice experience when I can help out the biker who rode in after I did and in return hear about some of their adventures and perhaps be invited to share some of their stuff in return. But I'm also very comfortable spending the day with just me and a good book.
I like how the speed of bicycle travel allows me to soak up the landscape and impressions so much more and better than if I rushed through by car or train because I'm in the middle of it all and don't just watch it go by.
I enjoy the physical challenge too. It's very satisfying to be able to say: "I DID this!!!" and then see my sons bragging about me with their friends.
And - you can't start too early! - I'm working on my bucket list (See the Pacific Coast, do the TransAm, maybe enjoy a tour through Europe, and so forth). So many places to see and so many people to meet.
And finally, I haven't come across another cyclist in, or near, my neck of the woods who is as crazy and also looks to really, really load up his or her bike so it'll be that much harder to pedal up the hills for days, weeks, or even months at a time, just to experience all the above... Besides, going on a tour is the best way to meet other crazies like you and me and SlowAndSlower!