Pragmatism vs Optimism

Had a long argument with my team lead – regarding estimations! [ Something engineers would continue to dislike till the end of times! ]

Today was the day of sprint planning – planning for what to do during next 3 weeks …
As we went through the tasks, Ganesh ( my team lead ) declared that we should finish X amount of task in this sprint.
After some quick mental calculations, I declared that it is not possible to finish with X amount of tasks during this sprint..
And this started…

It went something like this..

Me : “We can’t finish these many tasks in this sprint.”
Ganesh : “Vishwajeet, you create negativity every time!”
Me : “No, I’m not creating negativity, I’m telling the practical facts!”
Ganesh : “Devs shouldn’t say “It is impossible” even before starting work!”
Me: “Why are you confusing pragmatism with creating negativity? These are two different things!
We must get practical with the facts and accept those !”

And it went on..

(It ended up with both of us eating Amul Chocobar, but that’s a different story.)
As it turned out after detailed estimation calculations, I was proven to be right.

This has been a confusing scenario for me.
Was it a negative thing to state something is impossible up-front?
Was it a negative thing in terms of being a good team player?
Or, was he correct, being a leader, to discourage me from saying so, in order to keep up with optimism of the team?

Is it pessimistic to pragmatically state about these things before starting to work?

Or, in general,
Is pragmatism pessimistic?