The disease,

A strange epidemic kill all dinosaurs.

There doesn't exist any virus, microbe or germ, able to kill a whole 'family' of animals like dinosaurs!

The climate change,
Earth becomes colder, dinosaurs die because of that cold.

Why dinosaurs should disappear because of cold, when tortoises, crocodiles and sharks stay alive, especially if dinosaurs like mammals were warm-blooded animals! There existed dinosaurs living in artic region, so able to bear cold climate!
This theory of a climate change, is not the good one, as a consequence, theories which implies that climate change can't be right too: The meteorite theory,  volcanic explosions etc.. etc...

The competition with mammals

Mammals destroy dinosaurs eating their eggs or eating dinosaurs themselves when dinosaurs are asleep!

We have always the same problem. May be we are stubborn, but this theory can't explain why reptiles survive when dinosaurs disappear.
 

The volcanic explosion,

Volcanic activty creates clouds upon the ground, sunlight can't reach the Earth, the result is a climate change.

This theory can't explain why dinosaurs disappear and not reptiles, the same weak side than the theory of climate change.

The meteorite theory

A meteorite reaches the Earth, it implies a  climae change.
 

This theory can't explain why dinosaurs disappear and not reptiles, the same weak side than the theory of climate change.