I look it as a test to see how relevant you are in their life.
If you're relevant they'll text good morning even if you don't text first for a change.
If you're not relevant they'll only ever text good morning in response to your good morning.
.
It may seem silly but it's such a simple courtesy and takes 5 seconds of your time.
If someone can't bother to send a good morning text first they're likely not that engaged in your relationship.