Do pilots reject takeoff if aircraft doors are not properly locked down?
The crew has already secured the door just before leaving the gate, cross-checked them and the pilots saw the sensors indicate closed. So if the warning goes off during take-off roll, it is not somebody's simple mistake, but something is broken. It might be the sensor, but it might be something on the door and that might be pretty bad. So as the old saying is “it's better to be on the ground wishing to be in the air than in the air wishing to be on the ground.”

Below about 80 knots the pilots will (should) reject the take-off for almost any warning or abnormal indication they get. If the problem can be fixed easily, they can just taxi around and take off a couple of minutes later. Above that speed the emergency braking is likely to overheat the brakes, so from about that up to V1 pilots will only reject for engine failures and other very serious warnings, while for other issues they'll make the circuit and return to land again.