Several conditions contribute. Distance your shower is from the hot water heater, type and age and size of pipe to your shower from your hot water heater and climate. Cold weather climate tends to cool the static water in the pipes while waiting to be turned on. Old pipes have a build up of rust / minerals inside slowing the normal flow. Distance, well it takes a while for the hot water to leave the tank and get to the tap especially if the other two factors are involved. Insulation of the pipes will alleviate some of this issue. This will help save energy and keep the water in the pipes hotter longer. If the water still takes an unacceptable time to warm up you have a circulation pump that circulates water from the hot line to the water heater through the cold line, installed. This means that you’ll have hot water at the hot tap immediately.