There can be many mechanisms for this effect: One possibility is that your friends say to you something like -- you know, they adopt a behavior that spreads to you -- like, they say, "Let's go have muffins and beer," which is a terrible combination.  But you adopt that combination, and then you start gaining weight like them.
この現象にはいくつものメカニズムがあると考えられます ひとつは友達がある習慣を身に着けて 自分にも広めるという可能性です 例えば「マフィンとビールで一杯しようよ」などと誘われて 最悪な組み合わせですけど でもその習慣を自分も取り入れて 自分自身も友達のように太り始めるのです