- limited availability of freshwater - obviously this varies according to the weather and the overall continental climate,
- the use of resources to get the water from the water source to your shower (pipes, water treatment, electricity, etc),
- the use of resources taken to heat the water.
The extreme version of a short shower is the 'navy shower'. This is where you turn the water on get wet, turn the water off then lather up, then turn the shower back on just to rinse off. Bit extreme and at present here in South-east Queensland we aren't at a level of water in our dams that would require this. However, if you're really into making a change, this perhaps is something that you might consider.
I think the current recommended length for a shower, that isn't consider too excessive or resource hungry, is 5 minutes. Certainly if you're not washing your hair or shaving your legs every day this is achievable. And if you're thinking the first 30 seconds or minute is when the water is heating up, perhaps the use of a bucket in your shower to catch some of this water and either use it to flush your toilet or use it on your garden.