a norm or a must

Is it a must to buy back souvenirs for your colleagues when you're back from business trips or vacation? The answer is vague. Well, as for me, it depends on my mood. When i feel like bringing back some souvenirs, i would. If not, i will not even bother to think about it. Therefore i dun rant whenever i don't see any souvenirs from other people. As for hubby, he never fail to bring something back for me each time he returns from somewhere. So, my conclusion is, it's not a must, it's just a norm.