Theres loads of different reasons, and as we can all be so different we have different foods we stay away from, some things I can't eat other Crohnie's can eat, it can be quite individualistic.
For me I have to avoid high fat foods as my body can't absorb fat too well which will cause horrible D. One tip I picked up for this was to take calcium supplements when I eat something high fat as it binds with what doesn't get absorbed and so lessens the D and the burn! When I'm flaring I also get narrowings so I can't eat fruits and veg or anything too high in fibre as it gets stuck which could then cause a blockage.
A lot of people with Crohn's also have intolerances, lactose intolerance is quite common so for some cutting out dairy products really helps.
On the day my doctor diagnosed me with CD 9 yrs ago he told me not to eat raw fruits, raw vegetables and cereals. I followed these guidelines for 6months then once in remission I slowly began eating them again. When I have a flareup now I still go back to these food rules. Everyone is different and can tolerate different foods. You will soon learn what bothers you. I don't get pain either but definetly the diarrhea when I eat stuff I know bothers me. For me I definetly stay away from artificial sweetners, diet sodas,and chewing gum even on a good day.