I've been an active person all my life and have been the same weight since I was 18 years of age through continual exercise and moderation when it comes to diet. Weight gain can be a consequence of health and glandular problems, and I sympathise with those who suffer through no fault of their own.
In today's society though I think many many people succumb to pressure - pressure of fast food, sweet commercials, too much choice in the supermarket...and so on and so forth. Almost subconsciously people become overweight! How many times do you hear the statement...'I only have to look at a cake and I put on weight!'
I wrote a hub on this very subject. Common sense tells us that, if you eat the same amount of food get through the same amount of exercise and still put on weight you're doing something different, or wrong!! For example you may be eating the same amount but at a different time of day eg piling the calories on in the evening instead of the morning/early afternoon when there's still time to burn the calories off.
It can be a complex thing because we all have so much to fit into the day! I think the secret is to really know your intake, list the foods you eat, the amounts, and when you eat them. Do this for a week or two then try and lessen the amounts or change ever so slightly the way you cook the same foods. Get a friend or relative to help you monitor the situation!