Just like location, location, location for real estate, water is the key to success for your lawn.
I don't mean to over-simplify the process. A LOT goes into establishing and maintaining a strong healthy lawn. But 9 times out of 10, improper watering practices cause lawns to fail. Most frequently that means not enough water at seed germination and then again in the height of summer. However, over-watering can also be a problem. This time of year, during lawn seeding and repair, you do not want to flood your new seed and grass, but rather you should just keep it damp. During days with no rain 20 minutes per day for each area should be adequate. After 3 weeks you can cut back to 20-30 minutes every other day.
You can see below how our new 1 year old german sheppard rescue is enthusiastic about lawn watering...