Myanmar has a tropical climate, and during the summer months monsoons can hit the nation. Spring brings excessive heat: From May to March, temperature can hold at around 40 degrees Celsius (over 100 Fahrenheit). Summer is the rainy season; a chart showing rainfall in Myanmar's current territory from 1901 to 2016 showed rainfall skyrocket during summer months to more than three and a half meters of rainfall for the months of June, July, and August. And in case you read that wrong, that's three and a half meters per month! The months of November through February are surprisingly calm, with low rainfall and temperatures in the (Fahrenheit) 60s and 70s. It should be obvious that unless you are fine with dying of dehydration and heatstroke, you shouldn't visit in the spring. Whether you go in summer or winter depends on how crowded you want it to be. Winter has the best weather, so obviously that's when most tourists go. If you don't like crowds, the rainy season is still very manageable.
Sources:
https://www.roughguides.com/destinations/asia/myanmar-burma/when-to-go/
https://www.responsiblevacation.com/vacations/burma/travel-guide/best-time-to-visit-burma
Myanmar is a very mountainous country, with not very much development. This can make car travel difficult.