Ask HN: How do you determine which database(s) to use for a project?

This is probably a stupid sounding question, but it really confuses me. How do I decide whether to use MongoDB or Redis, Redis or MySQL, PostgreSQL or MongoDB etc. for a specific project? Is it just a matter of taste or are each one optimized for certain use cases? I've heard of Redis being good for a cache, but then I hear some people just using it as their database. Which should I use it as?