The death of my wife.
And I know that some may find that weak of me or whatever but trust me when I say it is not. In fact, it is quite the opposite. First and foremost because we as mortals typically do not know what lies beyond, although I personally do have seen hints of it, glimpsed enough of it for my mind to still make sense of (the human brain, even though a wondrous marvel of evolution still is severely limited in what it can and cannot comprehend).
Additionally, anyone that would judge me for it simply does not know the bond I have with my wife. We are, to each other, the only thing that makes life worthwhile. Both having had a very complicated past and still facing numerous challenges in the present.
It is as simple as that... if she were to leave this mortal coil, I would literally have nothing left to live for. No career to speak of, no children, no one that depends on me being here. So, I would simply prefer to follow her. She needs me as much as I need her. Both in the mortal realm as well as in whatever lies beyond.