Worried that friend has changed so much

My best friend recently began dating a girl he works with, and things seem to have changed between us since then. He spends as much time with me as he used to, but often seems distracted and has begun worrying about his career and future a lot. He says he is in love with her and wants to marry her, but they have only known each other for five months. I haven’t said anything critical to him about this, but I am very concerned because I don’t know why he is behaving in this manner. I am also afraid that he may make some drastic decision that alters the course of his life and I feel as if it is my duty to pull him back a little. What should I do?
You don’t have to feel as if you are ‘pulling him back’ or being critical. If you have concerns, it is your duty as a friend to voice them. You may not be able to speak about his feelings and how genuine they are, but you don’t have to stop yourself from telling him what worries you. He hasn’t done anything yet, which is a great thing, but telling him how you feel as early as possible makes it easier for you to be more direct if he does do something genuinely worrying. Having said that, these are adults we are discussing, so you should also keep in mind that he has the prerogative to do anything he feels is right for his future. As a friend, you should be able to offer support if things don’t go his way.

I want to tell this girl how much I love her but can’t say anything because she happens to be related to a guy who hates me. He will never let her go out with me. Is there a solution?
Why assume she doesn’t have a mind of her own? Why not be honest, tell her how you feel, and let her draw her own conclusions?