So one of my headcannons for them is that they started their “relationship” when they were teenagers, it was short and never went beyond any serious physical thing, but still was very meaningful. Dipper (the one who thought at first his feelings were unrequited) was the one who decided to break it, after being caught by someone in school. This person ( a friend?) decides keep the secret but BEGS to please reconsider what they are doing, to think how their parents would feel, how society would look down upon them and most importantly how hurt and sad Mabel would get at the end because of those things. Dipper thought a lot about it before (he is Dipper, he thinks of everything) but is in that moment that TRULY sinks in the mess he brought Mabel into. So, even if it breaks his heart to pieces, he ends it, because he loves his sister so much to actually see her suffer because of him. He doesn’t realize she can endure that and even more because she loves him too. Ok that’s it.