why is human not satisfied with what they got?? why can ppl feel jealous so easily?

i don't understand this statement. why is human always not satisfied with what they got? i gave him what i can. and he feels it's not my best yet. but i feel i gave my best already! then i said i feel u're not giving me ur best also. he said he gave the best already. can anyone tell me what is wrong? why is human so greedy? i hope my relationship with him will get better because from what i feel we can solve this problem if we really discuss it properly. why do i feel jealous when he talks to girl? i know that i'm not supposed to tie him up. he's not my husband or what. he's just my bf. but i don't like the feeling when he talks to other girl. u know.. he will flirt. eventhough he say that's not flirt. but i feel he is flirting. arghhh! i hate it. why must there be jealousy in me! oops! i just cracked my head bone =P back to topic. that sunday when i went to queensbay with my friends to celebrate wei birthday, he said he don't want to pass his working place. i said ok. then i said i wanted to go see his working place and i asked him to stand here i walk pass awhile. i don't know why but he's following. so i said for the second time. he said never mind and he said he want to prank his 'friend' which his my band senior. ex band senior. i said it in a very like don't like it way. and the next thing i know is he walked to his working place as if there's a pot of gold in front there! i was so angry so i walked away with my friend. lik WTF! he said don't want to pass and he's walking there so damn fast. that time i was really really angry and my leg hurts. after 'flirting' with his friend, he came to me. i did not answer any of his questions. at all! i can't believe it. he's flirting in front of me! haih. this is not about the jealous stuff but still. if i'm not angry of this then i think something should be wrong somewhere with this relationship already rite? hehe. hate it so much! arghhhhhhhh.