When we speak positively, we feel positive, and when we speak positively of others we spread that positivity. Sincere authentic compliments generate positive energy in relationships and help reaffirm other's value.
The key to giving a good compliment is to compliment something they can control. Instead of complimenting someone's eyes consider complimenting the way their clothing choices bring out the color of their eyes. Compliments that go beyond appearance are often more appreciated than those that only note obvious external traits.
In addition telling someone what you like about them - even if you and that person do not see eye to eye - can help create feelings of forgiveness and understanding between you.