Metta practice is discussed by Kevin Griffin in One Breath at a Time:
Of course, it's one thing to be compassionate to strangers, and once we get in the habit, it's not that hard. But what about those we love? With them it's more difficult to disidentify with our feelings. And this points to one of the paradoxes of metta practice. Typically we think of love as a very personal, intimate expression of human emotion. But with metta practice, we move to a different level of love, to the impersonal, unconditional level, the level that's not responding to the other person or being but rather drawing from our own, inherent quality of heartfelt caring and openness. While our intimate relationships are typically reciprocal arrangements, in which we cosupport each other, with metta, there is no expectation of a return for what we give. In fact, with metta, we discover that the giving itself is the return; it is the reward.
Metta, lovingkindness, is a rich and layered practice. Exploring its boundaries - expanding the boundaries of our own self-imposed limitations - is an ongoing practice. I include metta in virtually every period of meditation I do. I practice metta for myself, for my family and friends, my teachers, politicians, the hungry, sick and dying, for animals, bugs, fish, birds, for the earth itself, and for the entire universe. The universe is a great sponge of metta, absorbing all the love we can give and giving it right back to us.
What I want to assure you of here is how metta can actually help you feel better. If you are feeling down in any way, or irritable or resentful, try doing metta. You will feel better very quickly - I promise!