I'm a little late to the party and you've already received a lot good advice.
I like building rifles for a couple of reasons, customization, and I like working with my hands. I used to be a cabinetmaker, and loved building models as a kid. Even as an adult I used to build model RC cars. It's a matter of working with my hands then enjoying the finished product.
So, building things relaxes me.
I'm currently putting some money together for my next AR. I'm going to start with a Battlearms upper/receiver combination.
Going to start with this;
http://www.battlearmsdevelopment.com...r-receiver-set