Unfortunately, there's no straightforward answer here. Hot flashes at night can happen in pregnancy, perimenopause, and menopause. Climate and weather can obviously play a role, as can the layout of your house. If your room gets a lot of sun in the day or has poor ventilation it will stay warm even at night. Sometimes it's as simple as sharing your bed with your partner - two people under one cover can generate a lot of heat, each making the other even warmer.
Often, however, it's just personal preference. Some of us just happen to sleep hot and need extra help with sleeping cool.