Player’s Handbook 1, p. 170, "Concentration" – "Spell" says:
"If you are affected by a spell while attempting to cast a spell of your own, you must make a Concentration check or lose the spell you are casting."
Now I have the feeling I played DnD wrong for years – this means if I buff myself with e.g. "Mage Armor" and try to cast "Magic Missiles" the next round, I have to do a concentration check, correct? And someone I buff with another spell, too?
There is no mentioning of the spell affecting me being from someone else or hostile.