Why is Metta Practice so Hard?

I find it very difficult to practice metta. I feel completely agitated when I sit to do metta even for a short period of time.is this normal? First I can't feel any metta being generated and I feel like I'm just repeating empty words to people I'm really not quite fond of to be honest.metta practice is unbearable i feel the urge to rub my face on sand paper when I do it.I use to think metta was an easy meditation to do as I hear people do it before their main meditation but how can this be,when it so difficult..I don't buy it. Also i found out metta means friendliness and i think this is the problem as I may not be born with this quality.My question is Is metta supposed to be hard? If yes then why is it hard?