I run Bitcoin Core (with limited incoming connections) and Armory. It takes a big one-time download that took a few hours, ~50GB of disk space (with plenty free), ~650 MB of RAM out of 16GB, ~1 CPU hour out of 400 (based on current uptime and CPU time usages), and minimal bandwidth requirements. The last few of those I can pause any time I want to reclaim the extra resources.
In return, I get to use a good, secure client (Armory) that's connected to the network independent of any external service or undue reliance on peers (to tell the truth about the state of the blockchain, or to protect my privacy). And having powerful local clients, instead of overly-simplified ones, helps me learn more about the technologies behind it. I also like helping secure the network.

For me, that's an agreeable trade, so I run a full node. For some people, the requirements are relatively larger, and the rewards are less important to them, so the balance does not tip in the "run a full node" direction.

Self-interest can, in fact, be sufficient, including in my case. Altruism is a small part of why I run a full node, but is not sufficient nor necessary in my case.

And I know how average users can hurt the network. If we had fewer average users, I'm sure I could bump up my max number of connections substantially. I have to keep it low because I'll occasionally have someone want to download a huge number of blocks from me, and I have little upload bandwidth, so it interferes with anything else I'm trying to do.