Doc not being more heartbroken makes no sense

If you were Doc, wouldn't you be a bit more heartbroken to not only know that you just lost a dear friend forever, and you'll never see him, but also that he's going to go from 'heavenly life' to a 'hellish horrorlife', where Doc is dead and his mom is an alcoholic and dad a wimp?

Even if we don't think about the 'Marty 1 and Marty 2' and how they switched timelines between each other, it should still bring a tear to Doc's eye. But he's fine, somehow.

Think about it; sure, Doc and Marty first met in 1955 (except from Marty's point of view), but that was like 28 years ago (I am assuming Marty is 16 and they met when Marty was 14, but this detail doesn't matter), so the memory is going to be hazy at best.

Do you remember anything you experienced 28 years ago just a few hours during one week vividly, when you are in your sixties or seventies?

Then Doc meets this OTHER Marty, the youthful chap with full of energy - however, his heart is heavy, because he can't tell this new friend his secret about the other version of Marty he has already met. So their friendship is not going to happen the same way AT ALL it originally did, because now Doc has this deep secret..

(though what kind of a friend was Doc in the FIRST place, to keep his whole timemachine-building project and plan secret from the original Marty for years, and how could he hide it so well? Does he have a Sekrit Lab? (2 euros to anyone that knows this reference without cheating))

Let's say they become almost as good friends anyway, somehow. Now, Doc has this great friend and a few years go by, and he knows the Libyans will come at him and the Marties will swap.

Since the lives of the two Marties are, by necessity, VERY different, their personalities, memories, attitudes, and maybe even luck are going to be different as well. This means, their friendship is also going to be completely different than it originally was. So the Marty that comes back will be more 'alien' to the Doc, he only hazily remembers him, but the Marty he LOSES FOREVER, is his TRUE friend, that is fresh in his memory.

Now, when this true friend goes past in time in the DeLorean, he won't be coming back, but the Marty from the other timeline appears. They don't have shared history anymore, the other Marty and Doc had gone on very different adventures, so their memories don't match. Basically, they are strangers to each other.

Marty's personality is also going to be different, and he shouldn't even have a skateboard anymore, since he has that truck he would DEFINITELY have been using to meet Doc in the Lone Pine Mall.

Everything would be so incredibly different, Doc really did lose a completely different friend than he gained. Sure, they will rekindle their friendship that only Marty remembers the way it happened originally, but Doc should still shed a tear for the different personality-Marty he will NEVER see again. They went through so much together, now he has a friend he NEVER did anything together, except the 1955 stuff, which mainly consisted of trying to get him back to 1955.

It's remarkarble that Doc simply won't save the Marty from this destiny simply by NOT showing him the time machine at all, but that's another thing that makes no sense.

In any case, movies don't seem to be able to make much sense no matter how iconic or classic they are.