I have a mac book pro 15 retina from work and a personal laptop of the same model. Stickers are a fun/easy way to tell my laptop apart quickly. There are numerous types of stickers also, I have several from different entertainment series that I like, some tech based stickers, and stickers for environmental or political causes I care about. I've had more people than i can count ask me about the environmental ones in particular. For example I'm strongly opposed to the building of the pebble mine in Alaska* so I have an anti pebble mine sticker on my personal laptop. Since I'm commonly working on side projects with friends, often in coffee shops or cafes and I tend to go to a lot programming related meetups I have had numerous people ask me about it and I take it as an opportunity to educate others about the issues involved and why I believe how I do. It's a form of low level/mild political advocacy for me that I've found to be extremely effective. This does not seem childish at all to me.

*For more info see here: http://www.savebristolbay.org/ and http://en.wikipedia.org/wiki/Pebble_Mine