Why are people jealous?

IMO ppl that are jealous are ones who have low self esteem.....if one feels good about themselves and is at peace with who they are and their life then jealousy isnt something that comes into play.....I used to be a jealous person..wanting what others had etc but then some yrs ago I clued in and realized what and who I truly am and I'm happy with what I found/saw....These days I have absolutely no desire to have what others have simply because they seem so unhappy with it ya know and I wouldnt want to give up my happiness etc for any thing

because for some reason they feel inadaquate to others.If a partner cheats,the betrayed one feels like the "other" person must some how be better than them,when it's actually a matter of character and the cheater usually has none.