Rafael asked:

What's the most important matter: love, good, truth, happiness? And, what is happiness? And love?

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I think love is the most important thing that we should want because it lies deeply at the source of
human need.

The importance of love can be seen in the following excerpts from Rollo May's book Love and Will:

Hate is not the opposite of love; apathy is...being uninvolved, detached, unrelated to significant
events...The interrelation of love and will inheres in the fact that both terms describe a person in the
process of reaching out, moving toward the world, seeking to affect others...and opening himself to be
affected; molding, relating to the world or requiring that it relate to him...Apathy is a gradual letting go
of involvement until one finds that life itself has gone by.

If it is true that the opposite of love is apathy, then without love there can be no happiness or interest
in good and truth.

Love as a form of being able to engage deeply with others is necessary to happiness. Rollo May says
that most of his patients in analysis have not found love and the consequent emptiness they feel
means that they become unable to love. They enter analysis in search of another source of
happiness.

Plato's Symposiumis devoted to the subject of love. Socrates asks "What is Love?" and quotes
Diotima:

He is neither mortal no immortal, but mean between the two...He is a great spirit and like all spirits he
is intermediate between the divine and the mortal...He is the mediator who spans the chasm which
divides men and gods, and therefore in him all in bound together.

There have been answers on happiness in these pages before which you can look at. I think that it is
the ability to feel joy.

Rachel Browne