It depends on the nature and depth of the relationship.
Jealousy usually means one person suspects their mate is not as "emotionally invested in the relationship" as they are. Therefore they don't completely trust them.
More often than not it has more to do with their past relationship experience where someone betrayed them and now they no longer trust anyone. Essentially they're carrying baggage with them.
In some instances people are jealous of having to share their mate with anyone (including their family and friends). I've also witnessed both men and women become upset because a person spoke to their mate and he/she was cordial in return. It's as if they believe their mate should never smile or be kind to anyone else on the planet!
That's not just jealousy that is possessiveness!
Most women don't trust other women around their man and most men don't trust other men around their woman.
Both genders can be equally jealous.
There's also those who believe if you don't get jealous it means you don't care about them or somehow you're taking them for granted!