why do we have to put the window shades up during plane take-off and landing?

Flight attendants ask that the window shades are open during the critical phases of flight (take-off and landing) so if there's a problem (like an engine fire, etc.) and an evacuation is required, they can quickly glance outside and determine if an available emergency exit is blocked due to a fire or other source of danger.

Another reason (which really only applies to day flights) is that your eyes need to be adjusted to outside light levels as best as possible. If you had to evacuate suddenly, you don't want to be exiting from a relatively dark interior to be blinded by the light outside. It is this same reason (but in the opposite direction) why lights are dimmed for night take-offs. Again, if you need to evacuate quickly, going from a dark interior to a dark exterior means less time for eyes to adjust and a quicker/safer evacuation - that's the theory anyway.

Better Situational Awareness for passengers. and for the most part better lighting for day take offs and landings in case of a incident.

It really is so that if the passengers are in danger, they'll be able to see it immediately and act accordingly. If the windows were closed, you wouldn't know what was going on until the captain decided to tell you...and what if the microphone didn't work...

So we can see if the wings have fallen off, or the engine on fire, or when we crash into the sea we can see all the fishes.

to see the out side world zoom in and out.