Why would a jealous friend try to sleep with your s/o?

One thing I’ve noticed that is a trend when someone is jealous of you or envies you, is that they will go after your boyfriend/husband/girlfriend or try to hookup with all the same people you do. Why do jealous friends try to sleep with your s/o? Is it to hurt you, push you down or to prove something to themselves? Or all of them?
Why would a jealous friend try to sleep with your s/o?
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