If lighting and composition are the technical aspects of phtography, then finding a good subject is the subjective aspect (for lack of a better term), this is because every photographer is different.
Some are interested in vibrant colour, others in monochrome. Some show harsh realities, others show fluff. Some shoot pretty girls, while others shoot gruff bikers. It’s up to you to find what you like, but I’ve come up with a short list of what to look for: beauty, interest and emotion.