Why my phone temperature high?

40° is no problem. According to HTC, 40°C is the maximum the battery should reach while charging (I strongly recommend to charge around 30°C though), if not plugged in it can reach up to 55°C without being damaged. CPU can reach over 70°C without being damaged. However, if your phone is getting too hot, you probably have some demanding apps running in the background. Make sure you close programs in the multitask oversight (close unused windows/apps), and use an app like Advanced Task Killer or Android Assistant to check out what apps are running and close those that are using much CPU/RAM and are not needed. Android Assistant probably easiest to set up/use, as it will allow you to automatically hide system services from the list of running apps.