1) It destroys our virtues. I have discussed this a number of times. I am sure all of you know. The Bodhisattvacharyavatara says:
Whatever wholesome deeds,
Such as venerating the buddhas, and generosity,
that have been amassed over a thousand eons
Will all be destroyed in one moment of anger.
Tsongkhapa has a long discussion here…
This first disadvantage of not having patience, we may not be able to see very clearly. The disadvantages that we can see clearly, are the following:
2) You will not experience peace in your mind.
3) Your happiness and joy will get lost. It obstructs your happiness, even in the future, because it stands against that.
4) You will not be able to sleep well.
5) Your stability of mind is lost.
6) If you have too much anger, you will even forget the people who have been kind to you before and all your friends will be fed up with you and go away; you will lose them one by one.
7) You will lose your friends. Even if you get friends by other means, if you have so much anger, they cannot stand you and you’ll lose them. According to Tsongkhapa these are listed in the Bodhisattvacharyavatara:
There is no evil like hatred,
And no fortitude like patience.
Thus I should always strive in various ways
To meditate on patience.
These are the materials on which you have to meditate. Meditating on these points clearly influences the individual and then the individual will begin to see what destruction it does, so you don’t want to repeat that. That is how it affects you, that is how it becomes living dharma.
~ Gelek Rimpoche, The Six Perfections, 2013, p. 53