Real Simple magazine has a helpful list of clutter causes. These causes have been covered here at Unclutterer, but this specific list is succinct at pointing out the causes and supplying solutions. From the article:
The obstacle: ‘If I get rid of this wedding vase, I’ll feel guilty’
The solution: People feel a responsibility to be good stewards of things, says Randy Frost, a professor of psychology at Smith College, in Northampton, Massachusetts, and a coauthor of “Buried in Treasures” (Oxford University Press). Especially items they’ve been given by or inherited from a loved one. Getting rid of a present feels like disrespecting the giver. But remember the true meaning of gifts.
“When you receive a present,” says Maxwell Gillingham-Ryan, an interior designer in New York City and the founder of ApartmentTherapy.com, “your duty is to receive it and thank the giver — not to keep the gift forever.”
Guilt is a powerful force to make us hold on to gifts from others. Sentimental clutter is equally powerful. The “I might need it someday” cause is also covered in the list along with procrastination, belief in future value, and bill paying.