It started as a symbiotic relationship. Reddit had no good image hosting solution and it was a real problem. Before Imgur came along, you'd often find image posts where the link was dead, dog slow, or took you to a page filled with ads and malware. Imgur made Reddit better.
If anything, Reddit was the parasite because its users used a ton of Imgur bandwidth and by direct linking, didn't give anything in return.
Once Imgur decided to actually try to become a sustainable business, they had to change how that worked by trying to ensure that people coming to the site also saw ads. Then they started building their own community and working to become a direct competitor to Reddit.
At that point, the right biological metaphor is probably too organisms with the same food source in the same ecosystem, competing for resources.
Given that, it's no surprise that they are both working to sever ties to each other.
But that was back in a time when reddit really was a news-sharing site. Huff huff get off my lawn and all, but back in the day it was more akin to hackernews, and so the "problem" of hosting images wasn't a problem at all - you'd be linking to articles that had their own image hosting solution. Nowadays it's a heavy picture, meme, whatever browser. Makes sense that imgur is trying to take over that niche.
Honestly before RES came out the proportion of images on reddit was very low. Going back further before subreddits I remember it as largely just news articles and discussions, much like HN.
Before subreddits, yes, it was mostly links to articles. But pics was one of the first subreddits, and images made an increasingly large fraction of posts over time. I think RES was as much a response to that as it was a driver of it. If anything, I think the rise of smartphones was the big push. It's simply easier to consume imagery than text on a small screen like that.
If anything, Reddit was the parasite because its users used a ton of Imgur bandwidth and by direct linking, didn't give anything in return.
Once Imgur decided to actually try to become a sustainable business, they had to change how that worked by trying to ensure that people coming to the site also saw ads. Then they started building their own community and working to become a direct competitor to Reddit.
At that point, the right biological metaphor is probably too organisms with the same food source in the same ecosystem, competing for resources.
Given that, it's no surprise that they are both working to sever ties to each other.