Sorry Nancy, not sure I can explain it actually.
I like the solo bit because I'm self centered, and going with anyone else inherently involves compromise. Compromise on when to stop, where to stop, where to go. Since I like not having an agenda (or even a pre-planned route), that part I do get.
Beyond that, here's what I could come up with.
1 - pause and reset. Almost impossible not to get caught up in the day to day minutiae while in the middle of it. Stopping, trashing my normal routine and taking an objective (fresh) look can be invigorating.
2 - Reset. Absence makes the heart grow fonder. I've found that I have a new appreciation for how rich and full my life is when I live (even for a bit) without some of the things which I love so much... family, wife, work, home.
3 - adventure. Having not become repetitive about any of my trips, they're all new (to me) and interesting in part because I've not done it before.
4 - Riding. At the core of my getaways is spending some quality time on the bike. Don't ride as much as I might like (largely my fault through laziness, inertia or fatigue) and it's awesome to get a chance to focus entirely on finding those choice roads which bring that inexplicably free feeling.
5 - Tourism. I relish the opportunity to see some new things, do a bit of people watching and check out some new parts of the country. Gotta keep marking off those states on them map! (nobalance.com)