Jealousy is a strange emotion. Sometimes it just takes over. You can be going through a normal day, minding your own business and all of a sudden you see something or someone out of the corner of your eye and the green eyed monster hits.
You want what they have. You want to be where they are. You want to be who they are.
Jealousy reminds you of what you don't have. Sure you can probably be happy without it. You've been happy without it until now. But then you are reminded of that thing or that person that you desire and you feel like you cannot possibly live without it.
When I see scenes like this I am so jealous.
I want to be them, sleeping in the sun without a care in the world.