No, I don't think all inventions come about due to necessity. I believe many inventions happen quite by accident when people are working on something entirely unrelated. I believe many inventions start when someone has a flash of insight and follows through on it. I'm sure that things like silly putty, slinky, and etch a sketches did come about due to necessity.
Many inventions are thought of and created daily. Some inventions are created due to necessity and to make life easier for everyone. Other Inventions come about due to their inventor's desires. Inventions like the "Snuggie" and "Shake Weight" aren't necessary for people. Other inventions such as the light bulb make life easier.
Some inventions are purely mercenarial in their origin. They are the product of an inventor and their determination to make a profit. I guess it depends on how you define necessity. Was the automobile necessary, when there were so many horse around. Inventions require markets, and markets require innovation, this is how the innovation machine works.
No, not all inventions come about due to necessity, because some people just like to invent things. Most inventions come from people needing a reason, but other things are just from people who like to have fun. The one-man-band costume is a perfect example of an invention that is not really necessary.