Debugging is an interesting activity: while it’s work, it’s often similar to solving a challenging puzzle that requires exploration of what’s actually going on (not unlike a criminal investigation). In that sense, debugging has some features of (explorative) play, which I find quite motivating. Writing test cases is different: one has to write test cases to reveal problems, which then have to be debugged. Thus, writing test cases is work that leads to even more work, which is a lot less motivating.
On the other hand, each test case is passed at some point, i.e., there is a happy end to each and every test case. Think positive! 🙂