What's a common mistake that young people in relationships make?

Answer: projection/transference = unconscious contents are projected on outer object. This is what usually happens in relationships and creates enormous attraction. Then, a person attributes non-existing qualities to the loved one (politician, friend, teacher etc etc.). Result of that is a completely imaginary relationship which has no support in reality. Projection/transference is an unconscious process (i. e. it has nothing to do with conscious intentions of the ego) which is responsible for unhealthy idealization.

Explanation: unhealthy idealization (which creates infantile dependency) must be confronted with reality. This is what depth psychology calls "withdrawal of projection".

Taking your partner for granted to an unhealthy degree.

Taking your partner too little for granted.

Letting the boundaries slip around your relationship.

Complaining about your partner to everyone except your partner.

Turning on the passive-aggressive switch.

Explanation:

These are just the ones that immediately come to mind. Lemme know if you need something else!