The No. 1 problematic personality trait in a partner: Break up if you see it, therapist says—'they're only thinking about what they want'

There are various green flags that can show up in a courtship.

Clear and consistent communication is one, Jessica Small, a marriage counselor and therapist in Colorado, previously told CNBC Make It. Respecting boundaries is another.

Conversely, there are red flags it’s worth looking out for when you’re dating to ensure you’re not starting a relationship with someone who could hurt you or make you miserable.

In fact, the No. 1 trait that says it’s time to end things with a partner is “somebody who lacks empathy,” says psychotherapist and author of “13 Things Mentally Strong Couples Don’t Do,” Amy Morin.

Here’s why.