What makes you buy a brand (e.g. iphone), a product (e.g an anti-dandruff shampoo) or a service (e.g. an office printer) ?

People buy brands like they make friends (as Nichiketa Choudhary has written), just like our friends, brands shape our experiences.
They show up at work, at home, and everywhere in between.
People can build friendships with brands if both the brand and consumer can exchange personal information.
To maintain a friendship with brand need Trust, Influence, Self-Identification, Shared activity and Evolution.

Honestly, it all comes down to word of mouth and customer service.

@Ashish Mishra Thanks for your good question If you want to have generalized equation for knowing why do customers buy a certain brand over another one here it is and I will try to simplify it as much as I can: The value that the customer receives must be higher than the competitors offers exist in the market.
So the Customer Value is the difference between the perceived utility of the product or service and the price he pays Then Customer Value= Perceived Utility - Price -------------> (1) The perceived utility doesn't depend only on the functional utility of the product but also it depends on the emotional utility and the social utility as well * The emotional utility of the product is the emotional link between the customer and the brand or the product like being emotionally tied to the Mini Cooper cars * The social utility of the product is the social image of this brand or the prestige associated of the brand like buying Rolex with an incredibly expensive price because it has turned to be jewelery rather than a watch and the social image associated with it So, Perceived Utility= Functional Utility + Social Utility + Emotional Utility ------>(2) These 2 equations summarizes the logic behind the customer decision making process I hope this answer was clear and helpful to you.
Thank you and thanks to all the people participating in this rich discussion.

its the added value that a brand has over another , is what makes a customer to buy brand X rather than brand Y ,since he is either mentally or psychologically convinced that he is getting more out of the brand on various levels than the amount of $ spent

Agreeing to what Ms.
Ghina said let me reiterate it -Say you want to buy an office printer - You can buy Brand X as having brand X gives you a social impetus ( wow you use brand X) or 'cos you know X is technically a good product in the market or 'cos X has a great service network .
Generally it should be a mix of all three.
But given / assuming one criterion supercedes and exceeds the other - is there a general categorization wherein you give brand buying / product buying / service buying a priority ? ---- say something like a wrist watch which everyone will see you wearing ; I will go for the best brand I can afford irresepective of it being a chrono, digital, analogue or number of service centres around.

The need of them...

Satisfied customers are said to be the unpaid  advertisers of the brand . If you give good after sales service as promised at the time of the sale the people bying the brand would be loyal to the brand. They would also make their friends and relatives loyal to the brand. Avdertising is needed to make the customers aware of the brand but after that the brand has to perform in terms of utility and after sales service . Other wise if the brand does not live upto its expectation the advertisement could backfire.

Brand is important too because it must have something extraordinary which normal product doesnt offer, but selecting within brand is through some exception quality either being advertise or through word of mouth or have seen any collegue friend using it.

there are various reason to buy particular Brand or Service- Word of mouth, to maintain status in society & USP feature of that brand or service.

brand mean a promise, promise the customer to receive a value from buying your product , ask your self why i bought this japanese car? this is the brand screening , more specific why nissan cars ? this is a brand selection, brands represent customer perceptions and feeling about a product or service.