First of all, these drinks can really be a huge contributing factor when it comes to gaining weight. The reason being, these drinks tend to have a lot of calories due to the high sugar content. These obviously aren't beneficial calories for your body, but that isn't even the biggest issue. The main problem is the fact that you can eat food with the same amount of calories, but it will fill you up. When you drink a soft drink, or other sweet drink, you'll get a ton of calories and a crazy amount of sugar, but it won't fill you up. Inevitably, this will lead to you eating something else and overloading your calories. This is why it contributes a lot to weight gain, because it'll push you over your calorie limit without you really even being aware.
It's something you drink without thinking, or maybe you do think, but just assume, it's only a drink, how much damage can it do? It may not be something you notice immediately, but when drinking sweet beverages all the time on a regular basis, the weight gain will be slow, but consistent. By the end of the year, you can gain over 7lbs, and when you look back, maybe you ate decently, but you neglected to take care of the beverages you were in taking.
Many people actually depend on these drinks; they rely on them everyday. Obviously we don't expect you to quit, but keep it in mind. Try to slowly cut back on them by substituting water. Think of the negative impact it's having on your results in the gym, and more importantly, on your overall health as a human being. As you begin to substitute water more, you'll find yourself being drawn away from these sweet drinks. In time, you'll be able to quit them completely. When you do, you will see an improvement in results, and feel better on a day, to day basis.