Bicycle tires can go flat for a number of reasons. The most common reason is simply because of a puncture, which can happen from things like glass or nails on the road. Other causes can be from over-inflation or under-inflation of the tire, as well as riding on very hot days.
There are a few things you can do to help prevent your bike tires from going flat. First, make sure you always keep your tires properly inflated. Check the pressure regularly with a tire gauge to ensure they are at the correct level. Secondly, try to avoid riding over sharp objects or glass if possible. If you must ride over these things, try to go slowly and carefully to avoid puncturing your tire. Finally, on very hot days, try to ride in the morning or evening when it isn’t quite so hot out. This will help prevent your tires from overheating and potentially going flat.