Another possibility, though I am not so sure how aware people would be of this: if you differ in lifestyle from other people, you might grow apart (less time spent together etc.). Keeping the same lifestyle could ensure that you remain friends.
I think knb's "weirdness" and pjebys "loyalty" seem like good reasons.
Another reason could be that they feel discomfortable about you making another decision because it could imply that they are making wrong life decisions. So persuading you to make the same choice also reassures them that they have have made the right choice for themselves, thus removing a stress source.
Also, a very simple explanation (you kinda mentioned this with the different values) they might just be trying to make the "correct" decision for you. People differ in knowledge. They might just think they make better decisions than you do and try to make your life better.
Is it possible in reality to fire a single photon?
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Maybe you don't just make the picture diagonally symmetrical because then if you would integrate over all the probabilities you would get 200% (because you represent every situation twice)?