Starting a motor puts the highest level of stress on a direct on line motor. Very high currents cause quick heating in the copper which causes expansion. At the same time there is high torque trying to distort the motor. If a second start is attempted before the copper has time to transfer the heat into the steel then local hot spots can exceed the insulation break down temperature which starts to degrade the insulation. This erodes some of the built in safety factor. Since it is impossible to know exactly where the hottest spot is and what peak temperature is reached there is no way to know how much a set of start attempts actually damaged the motor. If that particular spot has a bubble (or void) in the insulation one set of 3 or 4 start attempts could reduce motor life by 50%. If the insulation is strong at that point then there may only be a 5% impact.