I don't think my friends should get married,
Why don't you think they shouldn't marry?
I believe that they are too different from each other
So, you don't have any concrete reasons like lies or cheating. You should keep out of other peoples relationships and let them be happy.
I think it's okay to ask questions when you are concerned about their love
The relationships of others should not be your concern. It is best if you do not get involved.