I'm following a diet and train regularly and still gain weight, why?

You must give more details about your diet, exercise, water intake, sleeping time, health condition, social condition to experience dictation to solve this qustion 

Gaining weight at the beginning of a new exercise program is quite common, but it is normal.

but you must define that the the weight gain is fat or water retention or muscle.

you must drink six or eight8-ounce glasses of water and doing cardio exercises3 to4 times weekly

What is your body composition? Is the weight gain due to increased,fat,water retention ,muscle or all3? That is why it is important to go through the fitness tests before you begin an exercise programme. Have your muscle,mass, bodyfat, bodywater etc checked and recorded so you can come back and check against those results at a later stage and thus know which one of them is the cause of your weight gain/loss. A bit of increased muscle mass and water retention cant be bad.

- Make sure your diet & training are good and enough. You may need to increase your training duration, or frequency, or you may need to modify your diet. - Get enough sleep.7-8 hours daily at night. - Don't cheat, don't eat high calories snacks or juices, or fat-rich meals. - Avoid exceeded sugar & salt. - Drink enough water. - You may need to see a doctor to exclude any diseases that could increase weight. - Have smart fitness workouts:1) It's better to consult your coach regarding your training program.2) Your body may need a break, or a surprising action. Ask your coach about this.3) Weight control is an overall result for a healthy lifestyle. Each weight is associated with a lifestyle. If you are gaining more weight that means your lifestyle is supporting this. Modify your lifestyle by increasing your physical activities, decreasing your calories intake, and balancing your rest, sleep, work, ...