Possibly the most important practice that we have, in life and in Buddhism, is mettā. The Pali word metta is translated usually as “loving-kindness”. It comes from a root word meaning “tenderly befriend”.

We can achieve high refined states of meditation, we can have deep and detailed knowledge of the suttas and philosophy — but if we are not using all that power to further the spread of metta throughout our interdependent world, we are doing nothing different than we do when perfecting our skills in sky-diving or yoga or jhanas or scholarly pursuits — it is all for self only.

Metta does not mean liking someone, or loving them in the worldly every-day sense. It means “not dwelling in aversion”. It means generating our kindness and patience towards everything, every person, and all of ourself — even the aspects we don’t like, or that we find unpleasant, evil, or disgusting.

Mettā is where we begin, it provides the ground out of which we grow compassion and appreciative joy. All four of the brahmavihāras have love as their undercurrent: they are about adhesion and connectedness to the other.
But it is a special type of love or friendship. It is highly influenced by the last of the brahmavihāras: equanimity. It is a love that is not permeated by craving, grasping and the need for reciprocity.
Delia Kostner
Mettā means a little more than just kindness. It is a penetrating kindness, a kind awareness. Mettā means we can coexist peacefully in a kindly way with sentient beings – both those voices and personae within us and with beings outside. It doesn’t mean liking them.
Mettā is being patient, being able to coexist with the pests of our minds [and those outside us], rather than trying to annihilate them.
Ajahn Sumedho
First we have to start with ourselves, so in traditional Buddhist style we always start the practice of mettā by having mettā for ourselves.
This does not mean we say: ‘I really love myself, I really like me.’ When we practise mettā towards ourselves, we no longer dwell in aversion to ourselves. We extend kindness to ourselves, to our conditions of body and mind. We extend kindness and patience even to faults and failings, to bad thoughts, moods, anger, greed, fears, doubts, jealousies, delusions – all that we may not like about ourselves.
Ajahn Sumedho
How can you like your enemies? But we can love them, which means that we will not do anything to harm them. We will not dwell in aversion towards them.
You can be kind to your enemies, kind towards people who are not very nice to you, who insult you and wish you harm. They may be unpleasant people whom you cannot like, but can love.
Mettā is not a superman’s love; it is the very ordinary ability just to be kind and not dwell in aversion towards something or someone.
Ajahn Sumedho
Mettā is one of the four brahama viharas that we practice: Mettā, karuna, mudita, upekkha.

Having loving-kindness (metta) towards someone does not imply that we like them, nor does it mean that we have to like them. The same for having loving-kindness for yourself!
It just means that whether you like them (or yourself) or not, you see their needs as separate from yours, and sincerely wish the best for their welfare.
What is the difference between metta, loving-kindess, and karuna, compassion?
We cultivate metta for all aspects of everyone, of the whole world, of the world inside ourselves. We send loving-kindness, and wish for other beings to be happy.
We cultivate karuna for the beings in pain and suffering. We feel for their pain, and we wish for them not to be in suffering.
The central chant of mettā for all beings -- sabbe sattā sukhi hontu -- means "may all beings be happy". It is an expression of the universal compassion that Buddhists aspire to.
Sectarian Buddhists who still use terms like "hīnayāna", or maintain that Theravadin Buddhists are not interested in benefitting all beings, may like to reflect on this phrase which is quintessentially Theravadin.