Is it rational to believe that human suffering should not happen?

No it isn't, it's a delusional belief. Minimising that suffering requires a substantial committment by the rich and powerful. They are too preoccupied protecting their own interests

Philosophically speaking, it is a normal expectation of every individual that he should not suffer at any cost, for any reason.

Two separate ideas, that suffering is bad (and bad things should not exist) or that humans must be able to prevent bad things. The first one is rational in fact to think any other way is irrational. The second is of course ludicrous and I doubt anybody thinks it.

All organisms evolve through suffering. It's impossible for any human being to not suffer either physically or mentally. So long as human beings die, they will suffer. And even if they lived forever, they will suffer.

Endless joy is not very rational either... pure bliss would get boring after a while. What's rational is a degree of happiness balanced by some suffering or dissatisfaction to provide the entity with catalysts for further progress and evolution.

Love and Light

Unnecessary sufferings should not be.

It's incorrect. It's borne out of people's choices.

There are ups and downs in life. Perhaps, it shouldn't happen, but it does and everybody is working on a piece of it.

Unnecessary suffering, yes.

Yes. And this world was once perfect and without suffering, until man decided to rebel and turn away from God. That's when wickedness set in and what has brought us this world now.

Not until sin is eliminated since sin causes suffering.