In a collaboration, like in a friendship or romance, one day you can just realize you are just going through the motions. The science may be interesting, but the collaboration isn't working anymore. Maybe you are doing too much of the heavy lifting. Maybe your interests or goals have drifted apart. Maybe the work of maintaining a long distance relationship is no longer balanced by the benefits, or the specialized knowledge your collaborator had is now available in your lab. At that point, it might be time to end it. Especially for me now, newly on the TT when I am trying to establish myself for tenure, I can't afford to have collaborations in name only. Sure, I get some extra papers (maybe) as a middle author from their lab, but I need to have papers that are from my lab alone, and that makes the bar for collaboration a little higher, especially for work that doesn't require specialized knowledge or equipment.
It is hard to let go. I find myself wondering if I made the right decision. In the short term, we are now racing to finish a set of experiments before they do something similar. In the long term, the door is still open to work together again, but I am pretty sure it won't be on this particular project. So for today, I am letting myself wallow in doubt and loss, and tomorrow I will pick myself up and get cracking on the science!