It's all for humans

Ask yourself, who or what is going to use the code I'm about to write.

1. You're building a UI:
   "A human" you answer. Certainly, that's the whole point of a UI.
2. You're building a service or API:
   "Other applications / systems" you answer. Directly, yes. But who's using that application / system? "Yet another application" you answer. OK, but who's using that one? "A developer" you answer, or "The sales group", or "A website visitor". What do all these "users" have in common? All humans.
3. You are fixing a bug deep inside some nasty code:
   "This stinks, I hate fixing bugs" you answer. Well hopefully you're making the code better to help out the next human (programmer) that visits that code, not to mention the human asking for the code to be fixed. 

Software is cool, but what makes it cool is its usefulness to human beings. Without being useful to humans, it is software for software's sake which is vanity.