They Start Acting Too Jealous
You often hear people say that jealousy is a bad thing. It seems like those who say it just repeat a cliche over and over but there might be some truth to it. Jealousy is a human emotion and everyone feels it to some extent.
Sometimes it can even produce positive results. Unless you are completely emotionless, you must have felt the sting of jealousy at some point in your life. There is evidence that jealousy exists even in animals.
Philosophizing aside, an excessive amount of jealousy in a relationship can cause friction. There is a golden rule when it comes to these things and that is if someone is cheating than the same person will be more suspicious of the partner they are cheating on.
So, if your partner suddenly has fits of jealousy it might mean that he/she is planning to cheat on you. It might seem strange but it certainly is true and it has happened countless times before.