What does it mean when a parakeet chirps?

Hi, I have a pet parakeet, and I am trying to make sure it's happy. When it's chirping does it mean that it's happy, or mad, or upset/sad, or excited? Or does it want something? One time my parakeet was thirsty, and I took away his water dish because I needed to clean it. It was chirping and getting mad and pecking where the dish used to be. Then it stopped when I replaced the dish. I'm guessing at the moment it was chirping because it wanted something. Or it was upset that I took it away. Anyway, sometimes I'm downstairs in another room (hes upstairs) and he starts chirping and squawking. Does that mean he's lonely and wants me to give him attention? Sometimes he stops when I get up there, sometimes he doesn't. Other times he will be hopping around his cage and just start chirping. When it isn't obvious why he's chirping, how do I know how he feels? Does squawking always mean mad/upset? and chirping happy? I don't know. Thanks!