i find that when working in the same company same department, it is better to has colleagues as friends rather than friends as colleagues.
friends as colleagues means that u introduced your friend into the company and might even work in the same department as u.
the reason is simple your best pal might make a good friend, but that doesn't means he will make a good colleague as well.
i experienced it myself. cos since 2 of u knows each others, there are 2 likely scenarios, the senior one will find it easier to arrow his friend to do work, and as his friend, the junior one don't have the heart to refuse and ended up doing all the work.
or it can be the other way round, the senior one is a nice guy, he don't have the heart to arrow his friend to do work or the friend will show no motivation in doing work since he knows u are his friend, u won't blame him.
these are just one of the many examples. if anyone have anything to add on, do share them here.