If we were to spread a bucket of clean water (i.e., metta) about the garden, it would bring joy to all. If we were to spread a bucket of filthy, smelly, contaminated water about the garden, we would hestitate, hold back the harm.
Or if we had resentment in our hearts towards some people, we would struggle to aim the bucket of clean water that way but not this way. We would exhaust ourselves in our stingy dispensation of clean water, struggling in resentment.
Resentment, ill-will and anger are toxic. If we hold these unwholesome things in our hearts, then we can't really invite others into our hearts without a little house-cleaning.
Consider the mildest of these, which is resentment. AN9.30 explains how to get rid of resentment.
Nine methods to get rid of resentment. Thinking: ‘They did wrong to me, but what can I possibly do?’ you get rid of resentment. Thinking: ‘They are doing wrong to me …’ … ‘They will do wrong to me …’ … ‘They did wrong by someone I love …’ … ‘They are doing wrong by someone I love …’ … ‘They will do wrong by someone I love …’ … ‘They helped someone I dislike …’ … ‘They are helping someone I dislike …’ … Thinking: ‘They will help someone I dislike, but what can I possibly do?’ you get rid of resentment.
This list of considerations is remarkable in its opposition to commonly held world views that uphold retribution as justified (e.g., witness the proliferation of revenge movies.)
Suppose we are driving with metta and someone cuts us off in traffic or honks their horn at us. At that very moment, metta tends to fly out of our hands as they clench into fists and our hearts fill with resentment. If we have any "chips on our shoulders", metta meditation is impossible. First we have to get rid of the resentment.