It’s unfortunate. Of late, many of our wisest and respected Rabonim have passed away. These deaths have occurred in close proximity. A number of others have been seriously ill, some remaining so, and others recovering Baruch Hashem. In almost all cases, however, the Rabonim have been old men; in some recent cases, over 100 years of age.
Nobody enjoys seeing their mentor seriously ill, or worse, passing away. It is understandable that feelings of sadness diffuse and displace an otherwise more positive disposition. 
The Vizinitzer Rebbe, a holocaust survivor, had suffered from Alzheimers and passed away at 95. He left a legacy of Torah and Chesed, and his sons and sons-in-law are considered paragons in their own right.
Yesterday, R’ Chaim Pinchas Scheinberg, who I met when he visited Melbourne some years ago, passed away at the age of 101. Reportedly, he was also experiencing Alzheimers. Sadly, R’ Yosef Shalom Elyashiv, who is aged 101, is still comatose in a critical situation due to life threatening illnesses. I hope he recovers quickly and resumes his earlier activities with renewed vigour. R’ Chaim Kanievsky who is in his mid-eighties, and R’ Aharon Leib Shteinman who is 100, I believe, and may they both be well, both were taken to hospital recently.
How should we react to this? I’ve noticed that almost all reaction is of the variety:
Oh Hashem, we have sinned, we have to do T’shuva … please stop this, we are sorry
Look, we have sinned. We also sinned while they were well, and all was good. We do have to do T’shuva. There is a tendency, however, for everyone to gratuitously assume that they know why Hashem does what he does. Why is it so clear to all that someone who is elderly passed away because of us or because of Tznius or Lashon Hora problems or … Is it heretical to say that Hashem has a plan which we do not understand. That plan includes a life span. Even Moshe Rabenu passed away. The physical body, short of a miracle, has a use by date. We all have it. Unless Mashiach comes, or Hashem decides to bestow an Eliyahu Hanovi style miracle on us, we all will eventually complete our task on this world. What about a different reaction?
Yes, we mourn. Aveylus is mandated by Halacha. Aveylus, however, is categorised by a period of silence, followed by positive discussion about the niftar and acts designed to elevate their soul in heaven. Is it anathema to thank Hashem for having had the opportunity to learn from, learn with, interact with, be led by … the great Rav who has passed away at the end of his life? Do we have to associate the death of an old person with our sinning?
The phenomenon is not just one with the Misnagdim. When the Lubavitcher Rebbe passed away, I recall the moment vividly. I was on the way to play at a Simcha. I was in a state of complete shock. He was, in my mind, one of the pillars of our generation. He and his father-in-law were directly responsible for my education. My mind was numb. To be honest, I didn’t at all feel like playing at a wedding. The MBD song “Mashiach Mashiach” was a great hit, and I recall singing it with a lump in my throat. I had no problem with the concept that the last Rebbe may have been a candidate for Mashiach, but now, after he had passed away, I sang that song in the knowledge that Hashem had chosen someone else. It was a negative feeling that was meant to be tinged with a positive thought. Not easy! I didn’t feel at that moment that I or we had done something wrong, that we had sinned, and because of our sins, this great Torah Sage completed his mission on our Earth. Clearly, the continuation of his institutions and mode of Yahadus is still felt, and in this way, he is “alive”. This is the positive side I was alluding to above. On Chabad blogs, instead of the “We have to do Tshuva” style responses, you get the “Oh, Mashiach Now, how long do we have to wait”. It is a different style of response, I admit, but again, it doesn’t recognise when someone has gone to another world because their “time is basically up” and they have completed their Shlichus (to use Chabad parlance).
When a truly great Rabbi passes away into the next world, his legacy remains. His teachings bubble along, and people stay inspired. We see this with the great luminaries, such as the last Rebbe from Chabad, R’ Kook ז’ל, the Rav ז’ל, R’ Moshe ז’ל, R’ Shlomo Zalman ז’ל, the Chazon Ish ז’ל … their indelible mark remains and seems to grow well after their passing.
In contradistinction, we have also witnessed the unspeakable tragedy in Toulouse, which hit me even harder when it was revealed that one of the Kedoshim, Gavriel Sandler הי’ד was named after my friend, R’ Gavriel Holzberg הי’ד who was murdered in Mumbai. True, these people “fulfilled” their Shlichus, but we are disturbed by their death in a profound way. We ask why their days were not filled. If they could achieve so much in so little time, how much more could they fulfil if they had lived to 80 or 90 or 100+? We do reflect on this, and some of us are inspired to improve ourselves (Tshuva) and others wring their arms and cry out for Geula (We want Mashiach now). These are “normal” responses. With this in mind, let us remember the Pasuk in Mishlei:
אין חכמה ואין תבונה ואין עצה לנגד ה
there is no wisdom, or understanding or counsel over Hashem
pitputim 