@Kim **********
international flights, you must show your passports. For domestic flights, it’s easier for the people at the counter to ask for the passport because:
1. They aren’t native English speaker and it’s easiest to remember one concept for ID (“passport”, instead of having to remember to ask for “driver’s license “, “Pink ID card”, etc).
2. Everyone should carry the passport with them at all times, and it’s a reasonable assumption to assume the person flying has a passport.
If you don’t have a passport, and you show your Thai drivers license, it would work too.