Hindsight is always 20/20. Many of us use this feature of our humanity to look back at how we treated others to see if there are opportunities to improve. More than once we find them.
Sometimes you might find that in areas where you thought you were being kind by saying yes you were actually hurting someone–sometimes that someone is yourself. Selfless acts are noble and if done properly they can help us stretch our skills and gain empathy for others, but all too often we say yes to things without thinking of the bigger lessons involved.
Sometimes kindness means no.