This topic comes up a lot, which makes me think that there has been a generational shift. I always thank people for gifts, and it's nice to be thanked when I give presents to others. But it does seem as though effusive thanks have fallen from fashion, and it causes so much heartache that I think it's time that older people realised that it's not personal. A text, or a message on Facebook is still a thank you, and I agree that if someone has said thanks when given a gift that covers it. Life is busier now, and there are so many ways of communicating, that the days of formal thank you letters are in the past. I have had some recently for wedding presents and baby gifts, but long after the event, as both are busy times for the recipients. For less auspicious occasions I usually get a thanks in person, a text to say something has been delivered, or sometimes a photo on FB with 'look what mum got me' or similar, and that's enough.
People get similarly upset about cards, and again, I think it's very much a case of times changing, and it's such a shame when people take it personally and see it as a lack of care, when it really isn't. Please try not to take it to heart. 
Hives , Can anyone explain the reason ?
