1 They don't get cold easily
Some people have bodies that don't get cold easily. An abnormal activity in the thyroid gland can make a person feel warm in winter. In such a case that person might wear summer clothes in winter.
2 They are used to lower temperatures
People who are used to lower temperatures , like some immigrates, might not find the cold temperature in some countries very cold. Those people might wear summer clothes because their bodies are used to harsher temperatures.
3 To show off endurance
Some people , especially teens, might wear summer clothes in winter in order to show off strength or endurance. In such a case the motive behind that behavior is purely psychological.
4 To shift between clothes
Some people wear summer clothes in winter then carry a jacket or any other winter clothing that they can put on immediately. Those people do so to shift between the different clothes easily based on the temperature. (See Why many people choose to wear branded clothes?)
5 To Differentiate themselves
Some people try to break rules and do odd things in order to differentiate themselves from others and advertise their unique identity. Those people do this because they feel better about themselves when the believe they are different.
6 They are trying to rebel
Some people, especially teens, might try to rebel against common rules and norms just to establish the identity of being rebels.
7 It's more comfortable for them
Some people wear summer clothes most of the time because winter clothes are not that comfortable for them. Those people don't like to put on many layers and instead they prefer the one layered summer clothing.
8 They have higher tolerance to cold
Some people have higher tolerance to cold than other people. Those people might wear summer clothes in winter just because they are not feeling that cold. (See Why some people prefer winter over summer?)