Why is my eye bloodshot?

For a couple of years my left eye has been mildly red and bloodshot. It is usually bloodshot in the morning, sometimes afternoon, and more frequently in the evening. Eye drops relieve it for a few hours at a time, but it's a hassle always having to deal with it. My optometrist has given me a bunch of different prescription eye drops to try but nothing has been a permanent solution. Every once in a while it gets very red and in contrast to my white right eye, it is quite grotesque. What is causing this?