From what I've been told and what I've read before, interviewers ask this question to 1) make sure you're not a narcissist/psychopath/sociopath who thinks they're perfect and better than everyone else 2) further evaluate you not only as a candidate for medical school but also as a fellow human being because NO ONE is perfect. We all have our strengths AND weaknesses. However, I know that adcoms would rather see candidates talk openly and honestly about any perceived weaknesses they have and what concrete steps they have taken to address/fix/improve upon their shortcomings.
I've also been struggling to come up with a good, honest answer for this question. This might be not be good, but I was thinking about saying something like this:
"I believe one of my weaknesses is that sometimes I have a rather pessimistic or negative world view because of everything that is currently happening in my life. However, I have taken great steps to improve upon this shortcoming of mine. One step I have taken is to always keep the big picture in mind. In recent months, I have told myself that even though the road to becoming a physician is long and difficult, with many specific details about healthcare, medical school, residency, and being a physician that aren't necessarily positive or enjoyable, the privilege of being able to help and serve others with medicine is still inherently worth it. Another step I have taken is to do my best to stay optimistic every day. At the end of every day, I think about what happened that day and write down 2 or 3 positive events that made me happy, made me smile, made me laugh, or made me continue to be optimistic for the future. I recently completed the 100 Happy Days challenge, which comprised of me taking a picture, uploading it to social media, and writing a caption that reflected at least one event that happened that day that made me happy. During those 100 days, I noticed that I found myself consistently having fewer and fewer negative thoughts or pessimistic feelings every day. When the 100 days ended, I found myself completely re-energized and ready to take on any challenges that each day brings forth. I find that I am now much more positive and optimistic about my world view on life, which I believe will greatly help me get through medical school even during the toughest days."