Re: "Why you put an engine on your bicycle?" and best answer goes to...
Just like Dan, it has been a childhood dream to have a motor on a bike since I was putting cards in spokes.
When I started high school we had wood shop, metal shop, and mechanics. I was able to take wood shop and metal shop my freshman year. My sophomore year they dropped all three programs due to budget. I always wanted to learn more about engine mechanics. These little engines are perfect. Sometimes they force you to learn things - like when they strand you 8 miles from home! They are simple to work on and if you really screw up, cheap to replace.
I also have really enjoyed seeing areas of town I have never seen or noticed. When on my motorized bicycle I take the back streets and find cool houses and such I would have never seen. On our bike meet last week I was in an area of town I have never seen, although I work within a couple miles.
It also fun to meet some of you through these forums. It's comforting to find people more obsessed with silly hobbies. This might be my favorite hobby yet! I am just living to ride my bike on the weekends. I can't wait to have longer days so I can go back to riding after work.
Lastly - Why wouldn't you put a motor on a bike?