This is a series of riddles, whose answers all have a relation to each other. Answer the riddles to find the relation.

Riddle 1:

When things have me
People are attracted to them.

I am said to be skin deep
And in the eye of the beholder.

Riddle 2:

Some say I conquer all
And I even have my own holiday.

I am given and received
And I can cause joy or grief.

Everybody wants me
And everybody hates to lose me.

Riddle 3:

I come around when a friend is not your friend.
People hate me.

I am the evil twin of trust,
And I can result in hate.

Riddle 4:

I am housed in large buildings,
Though you can find me all over the world now,
I was born in Italy.

I like to be sung,
And people dance with me too.