In my last long-term relationship (over two years ago at this point) it was when I started going to therapy (for like the fifth but final time). I realized that I had some problems I needed to deal with and I also realized that the relationship at the time had been established for about a year, maybe more...and had been enabling some of the harmful behaviors that I was going to therapy to stop.

I knew by the time I got out of therapy I'd be a different person (or at least hopefully). And I knew that even if I could stop doing the harmful things I was doing, at that point I would not want to be with someone who had previously enabled and even at times encouraged me to do these harmful things. I knew I needed to stop smoking 1/8 of mj every 3 days (to myself). I knew I couldn't be dating someone who smoked about that much if I wanted to quit.

I started therapy. I moved out to my own place after 3-4 months. We broke up about a month after I moved out. But we kept hanging out for a very long time, our interactions gradually tapering off as I developed friends and renewed friendships and so on. As I regained independence and began to prove to myself who I was. Now we haven't talked in over a year and I don't miss this person at all.

Hmm, as for the last person I casually dated, I knew it was over...well, I don't think there was a moment I "knew" it was over but I realized I maintained a certain amount of detachment the whole relationship through. Maybe it was when she wanted me to go see Mumford & Sons. I never cared about this person as much as I thought I did. I think I was just kind of idling along and settling, "content." We'd get into fights and I'd tell them, "I can just leave. I can just walk out this door right now. I don't need to be here for this." Ultimately I'd choose to stay. But I think that attitude should have been a sign.

Edit/Addendum: And I am sayin'. Nothing wrong with the good ol' devil's lettuce. But there is something wrong with smoking $60 of it every 3 days.