Why do I seem to miss my loved ones that have passed at the most random times, with no recognizable triggers?

This morning I woke up and I missed my grandmother. She passed away in December of 2009, so the anniversary of her passing isn’t anywhere near. Her birthday is in May, and I didn’t hear a song or see a sight that reminds me of her. I have just been fighting the urge to cry all day because I miss her today more strongly than most days.
This happens often with other loved ones that have passed away. My sister most frequently, but just last week I was moping around about a close friend that died last year. My grandfathers have both been dead for nearly 15 years, but I still mourn my paternal grandfather on a regular basis.
Why is my grief so cyclical? Will I ever be out of a state of mourning? Is this normal for most people, or is it possibly related to my having experienced so many traumatic compound losses in a short period of time?

Please don’t refer me to a therapist. I have a therapist. I want personal opinions, not professional opinions.