Why does my soap melt at the sink?
Soap is designed to melt when it comes in contact with water so that is cleanses your skin. So when a bar of soap is left on the sink, if there is water that accumulates around the bar, it will continue to dissolve the soap until the water has evaporated.
Once the water has evaporated, the soap will stop melting. To make your bars last longer, find a place that allows the water to drain away from the bar or get a raised soap dish that allows air to pass under the bar. This will make the bar last longer.