What are you currently trying to prove to someone or to yourself? If you could not identify anything consciously, think to see if you might be trying to prove something to someone or to yourself subconsciously, where you were so focused on the goal you might have forgotten the reason. If you could not identify anything still, is thinking about it sparking any ideas or motivation? I always have a list of things to prove to myself and/or others. My challenge is to remember them all rather than not have a thing to prove!
Having to prove something, especially when it is your choice rather than forced upon you, is among the easiest source of motivation you’ll find. That can be proving to yourself or others, with the better source dependent on your personality and/or the thing to be proven. In proving to others, you might leverage a grudge for motivation, which may or may not be healthy pending how you do it. Meanwhile, in proving to yourself, you can give yourself a chance to prove things nobody might ever care enough about you to doubt you so you can prove it to them.
Besides the motivational benefits of always having something to prove, the challenges also keep your hunger and zest for life alive. Combining those benefits, for things to prove of your choosing, why would you not want to always have something to prove, whether to yourself or to others?