If you went through something a year ago, and let's say it felt pretty lousy, when you reach the same time the NEXT year, feelings that were present only 12 months before often come flooding back. You forget what transpired in the time in between, how you got happier, stronger, even better than you were 13 months ago.
All it took to set you reeling was the nip in the air, the way the sun went down early, or the feel of the wool coat you finally pulled out of the closet. That's all it took to send you back.
I don't imagine it's quite the same for people from places like Florida and California, but here on the east coast, depending where you were one year ago, it can put a smile on your face, or a pang in your heart.