Fleas love sandy places and hiding under things.
Their eggs can survive for ages until vibration makes them hatch.
Warmer winters means that there's no proper die off so they manage to overwinter.
1. Your dog get fleas from hanging around sandy areas where other dogs (with fleas) also hang out
2.
Any flea treatment loses potency as the insects that don't die get a tolerance to that toxin.
For this reason, I recommend only using flea treatment if you really Have to,
and use different ones each time.
3. Vacuum all the dusty corners and bedding where flea eggs can be. Wash their beeding as well.
4. Wash the dog using a gentle soap and warm water. Firstly, make the neck soapy so all the fleas can't rush up onto their head when you start doing the rest of the body. Soap the entire dog from the neck back. Ideally have a person at each end and use a liquid soap so you can sponge the warm soapy liquid onto the dog and access all areas underneath and the tail as well.
Rinse well and towel dry. Try not to let them roll in dirt and let them back onto their clean beeding that has been drying in the sun!