Critical thinking entails asking who, what, when where and why. When a person gets into a relationship, it's important that they ask themselves
who is this person I am getting involved with
why am I getting involved with them
why would i marry them
When deciding which questions to ask, you should first ask yourself what am I looking for? What do I want out of this relationship? Then if you are thinking critically, ask yourself, 'why do i want the things I do?', and 'why am I expecting those things?'
Thinking critically allows us to think with our heads and not just with our hearts, to make sounder decisions based on how things are, not just how we want things to be.
I hope this helps, let me know if you want to talk more