I don't know if I can come up with 10 ways, but I'll try.
1) Unforgiving someone holds back the potential of future growth.
2) Negativity weighs on a person to the point of bad health.
3) Living in the unforgiving past takes away the beauty of the present.
4) Unforgiving someone can lead to hostility, which can turn to anger.
5) Unforgiving someone can create a less balanced life in regard to healing of the mind and health of the body.
6) Unforgiving someone allows them to have control over your life. It's a constant reminder of what they did in the past.
7) Unforgiving someone hinders the ability for a person to accomplish and achieve their goals because it leads to anxiety about things they cannot change from the past.
8) Unforgiving someone takes up brain cells that could be used to store new potential. It weighs heavily on the mind and takes up way too much time.
9) Unforgiving someone instills fear of the person and leads to a hesitant approach of future successes.
10) Unforgiving means you can't be free, ever, from the barricade that you placed in your mind toward that person. The road block to success hinders your outcome.