Your 7 year old kid is far more mature than what they were a few years ago. Your child can now exercise much more self-control than at the age of 4 or 5. They are able to now handle last minute changes and unexpected situations better and there are far fewer melt-downs. However your child is still dependent on you for building their self-confidence and this is precisely why they still become extremely sad when losing in something.
What you need to know
Although children this age are more focused on the outside world, whenever faced with a problem they will immediately look to their family for comfort and support. Most 7 year olds are actually very insecure about themselves and are not sure about their abilities. If they are working on a project and things do not turn up the way they wanted, then they will get easily disheartened. Losing at a game crushes their self-esteem. So you will find most children this age are bad losers.
Because of low self-confidence, most children this age are also susceptible to peer pressure. As they feel so unsure about themselves, their desire to fit in with the group makes them very vulnerable. Losing at anything in front of the peer group becomes a matter of pride.