I've responded to many pull requests with “Can you add tests?” Not because I'm a testophile, or to annoy the person involved.
When you send a pull request that fixes a bug, naturally, you must test it before submitting to ensure it actually works. Often, one thinks something can be easily fixed, but lo and behold, it ends up breaking even more. I don’t want to repeat myself, but by testing it, you created a test, so just attach it.
(Unfortunately, some people really don’t test their code. If it were up to me, I would give out monthly bans for pull requests made directly in the Github web editor.)
But that's still not the main reason: A test is the only guarantee that your fix will work in the future.
It has happened many times that someone sent a pull request that wasn’t useful to me, but altered functionality important to them. Especially if it was someone I know and I know they are a good programmer, I would merge it. I understood what they wanted, it didn’t interfere with anything else, so I accepted the PR and then I put it out of my mind.
If their pull request included a test, then their code still works today and will continue to work.
If they didn’t add a test, it might easily happen that some other modification will break it. Not intentionally, it just happens. Or it has already happened. And there's no point in complaining about how stupid I am because I broke their code for the third time, even though I accepted their pull request three years ago—am I supposed to remember that? No, so perhaps I’m doing it on purpose… I’m not. No one remembers what we had for lunch three years ago.
If you care about a functionality, attach a test to it. If you don’t care about it, don’t send it at all.