Quick chemistry lesson. Hair has a natural pH of 5.5.
Regular store shampoos range anywhere from a 4 to a 9 on the pH scale.
Why does that matter? I'll tell you.
Neutral solution: 7
Alkaline solution: 7.1-14
Acidic solution: 0-6.9
Shampoos that are more alkaline are more harsh and can make hair porous, brittle, and dried-out. A shampoo with a high pH causes color-treated hair to fade.
Shampoos that are a little acidic are ideal since they are closer to the natural pH of your hair and scalp.
Professional salon shampoos are highly concentrated, meaning you only need a coin-sized amount each time you shampoo. They are thick and let YOU add the water instead of filling half the bottle with it. On the other hand, store shampoos are often runny and watery and require quite a bit more to get the job done.
The professional shampoo will last a few months. The supermarket shampoo will last a few weeks.
The quality of ingredients really does matter. Likewise, the amount of quality ingredients also matters greatly. Some store shampoos boast they have the same ingredients as professional shampoos, but they never advertise they contain the same amount of those ingredients. Because they don't.
Non-professional shampoos are loaded with cheap fillers—chemicals that fill up the bottle for much less money. If you read the ingredients they won't straight out list "wax," but the long, wordy chemical synonyms will be there. That's why you THINK it's making your hair so "soft!" It's putting a crap coating on your hair that is building up residue. I can honestly feel it on people's hair in the salon. It's gross.
Business lesson! It doesn't cost them $3 to make the shampoo. That's what the store SELLS it for. Stores usually sell for double what they bought it for. Ok, so that's about $1.50. But guess what, the company didn't even spend that much money on the actual shampoo inside the bottle! There's the cost of the packaging, the cost of paying the middle-men to get it to the store and on the shelves, the portion they set aside toward advertising, paying workers, and on and on and on. What you've purchased is maybe, maybe 75¢ worth of fillers, harsh detergents, and lots of water. And you're putting that crap on your hair.
"But I Buy Professional Shampoo AT the Normal Store!"
Sorry to break it to you, but no, you don't. I know you can often find professional brands, like Paul Mitchell for example, on the shelves at your grocery store! Without getting too far into it, this is "diverted product." Paul Mitchell does not sell to chain stores and grocery stores. Scammers purchase real product, water it down with fillers, put it in near-identical bottles, and sell it to stores worldwide pretending to be certified retailers for the company. Cha-Ching for them! I could rant forever about this, so I'll stop there and write a separate post later.
That being said, there are some companies that started out professional (only sold by certified salons and professionals) but later decided to start selling to supermarkets and grocery stores. To skyrocket the amount of products they produce and to compete with other brands on the shelf, the quality often goes down. They add more fillers, which are cheaper for production.
With all this new information, please refrain from spending more on dish detergent than you do on your shampoo.