Bitterness in cooked asparagus can result from overcooking, tough ends, aging, varietal differences, and cooking methods like boiling. To minimize bitterness, trim tough ends, cook until tender but not mushy, and opt for fresh, in-season asparagus when possible. Seasonal asparagus typically has a milder flavor. Balancing it with ingredients like lemon juice, Parmesan cheese, or balsamic vinegar can also help reduce bitterness.