And that's really the key.
Folks who don't work with their birds/handle them, don't get this.
Birds locked in cages get angry, or territorial, or both.
Parrots are social by nature. They just have instincts that protect them from being eaten, that says "don't do it." So they are afraid. Once they realize it's safe, then they start to open up to you...
What you have to realize is that in order to do this with you, a parrot is quite literally trusting you with its life. Think about that one for a second... that's a lot of trust.
How often do you completely trust another human with your life?
The more they get socialized, the more social they become.
All living things respond to love.
It may not appear on any scientific periodical tables, but love is still an essential element... for all life forms.