Perhaps Pooh feels sad because he realizes that things are changing in the Hundred Acre Wood. The seasons change, and with that, some of the familiar things he loves might be different. Like when the flowers he likes to look at in spring are gone in winter. It makes him feel a sense of loss and thus sadness.
It could also be that he has a misunderstanding with his friends. Maybe he thought that Tigger didn't want to play with him anymore. Pooh is a very sensitive bear, and even a small sign of rejection from his friends can make him really sad.
One possible reason could be the loss of his friends. For example, if Piglet moved away. Pooh is very attached to his friends, and such a separation would make him sad. Another reason might be when he can't find honey. Honey is his favorite food, and not being able to get it could be a great disappointment for him.