I assume you're being asked to think creatively here. If I had to come up with points of similarity, my first one would be that both a guitar and a person need the right amount of tension to perform correctly. A guitar will not sound right if the strings are tightened too much, but it will also not sound right if the strings are not tight enough. You can argue that people are the same. If we have no tension in our lives, we have no urgency and we are not likely to work hard and fulfill our potential. If we have too much tension, though, we are likely to fail because we cannot handle the stress.
I suppose the other similarity could be that all of our parts have to work together for us to function properly. When you play a guitar, you have to hold all the strings in the proper way for each chord to sound right. The strings all have to work together (not to mention what your strumming/picking hand has to do). In our lives, all of our "strings" have to be "held right" as well. For us to function at our best, we have to have our work life and our personal life and perhaps our spiritual life all working correctly.