When we sweat, we lose not only water but electrolytes. The main electrolytes that are lost are  sodium (Na+), Potassium (K+), Magnesium (Mg2+), Calcium (Ca2+), and Chloride (Cl-), plus a few others. 
Notice the plus or minus? That means these are ions and hold a charge, which is important for a host of physiological functions, including maintaining pH (acid-base balance), muscle contraction, nerve conduction, and regulating blood pressure. So if we sweat large amounts from strenuous activities, it is important to replace these lost electrolytes. 

Note: after light exercise or activities (where you don't sweat much) you do not need to replace electrolytes. Drinking water will do the trick.

There are a few reasons why commercial sports drinks are not your best bet for rehydration.

First, if you read the label on a bottle of gatorade, the first thing you will notice is the high sugar content. While a small amount of sugar can be beneficial in boosting energy during exercise or competition, too much is unnecessary and can lead to blood sugar spiking then crashing.

Second, in addition to the electrolytes, there are often dyes and added natural/artificial colours. You body most definitely does not lose any artificial colours when you sweat!

The third point is actually no longer a problem. Gatorade and other sports drinks used to contain brominated vegetable oil (BVO) to improve stability. However, BVO is also a flame retardant that is banned in Europe and Japan. After a protest, gatorade has removed BVO since 2013.

The good news is that it's super easy to make a natural electrolyte replacement drink. There are a number of different ingredients and recipes that can be used, and feel free to experiment to find one that you like.
The ingredients that we want to include are:

Water / Coconut Water 
You need to replace lost fluid after intense sweating, so water is the base. Coconut water is another great option because it's a natural source of electrolytes. However if you don't like the taste, you can use fresh spring water.

Juice of Lemon/Lime/Orange 
Citrus juice is a fantastic source of electrolytes. Also, did you notice the citric acid ingredient in gatorade? This can be found in (surprise!) citrus fruits. You can also make your choice of sports drink flavour, like orange or lemon-lime.

Sea Salt 
Sea salt contains sodium and chloride, as well as trace minerals. Pink himalayan salt is a great choice.

Blackstrap Molasses
I like to add this as it adds some carbohydrates post-exercise, as well as a number of other nutrients and minerals. Some people don't love the taste, but a small amount mixes pretty well.

Honey/Stevia
I don't usually add this, but you can try it if you need to sweeten the drink up a bit.

1 cup water or coconut water 
1/8 tsp sea salt (i.e. himalayan pink salt) 
Juice of 1/2 lemon or lime (or both) 
Juice of 1 orange (or 1/2 cup orange juice)
1 tsp blacsktrap molasses
1/2 tsp honey to taste (optional)

Combine all ingredients in a large cup or water bottle. Mix well and enjoy!

"If the duck had a human mind, it would keep the fight alive by thinking, by story-making. This would probably be the duck's story: 'I don't believe what he just did. He came to within 5 inches of me. He thinks he owns this pond. He has no consideration for my private space. I'll never trust him again. Next time he'll try something else just to annoy me. I'm sure he's plotting something already. But I'm not going to stand for this. I'll teach him a lesson he won't forget.' And on and on the mind spins its tales, still thinking and talking about it days, months, or years later. As far as the body is concerned, the fight is still continuing, and the energy it generates in response to all those thoughts is emotion, which in turn generates more thinking. This becomes the emotional thinking of the ego. You can see how problematic the duck's life would become if it had a human mind. But this is how most humans live all of the time. No situation or event is ever really finished. The mind and the mind-made 'me and my story'  keep it going.

"We are a species that have lost its way. Everything natural, every flower or tree, and every animal have important lessons to teach us if we only stop, look, and listen.

Our duck's lesson is this: Flap your wings--which translates as 'let go of the story' - and return to the only place of power: the present moment."

- Eckhart Tolle, from A New Earth: Awakening to Your Life's Purpose