Reasons for low sex drive in middle age man in good health, and normal testosterone level?

Normal: Probably nothing physically wrong. Interests change over time. Quit pathologizing.

Still numerous: Still many things to think about such as thyroid, depression, stress, etc.

Many possibilities: Stress ranks high up there. In this day & age, worry about finances, mortgage, job security, etc can distract & decrease one's libido. Worry about family & friends can do so, too. If you're not happy in your relationship w/your partner, it's difficult to be sexually interested. Despite being in "good" health, bad lifestyle choices like smoking, excess alcohol & other illicit drugs can decr libido.