"Your stuff" could refer to a product, a service, or an ideology you are spending effort to propagate. Why buy from you?
- someone needs your product, and has difficulty obtaining it elsewhere (insulin)
- they like you and want to support your business to show the depth of the friendship
(network marketing exploits this dynamic)
- your selling format is so convenient (Walmart)
- they can't beat your price, or your value
- you do a better job of selling; you tell a better story (Obama campaign)
- your product is so neat, or has free features that are really neat (iPhone)
Other possibilities exist. Conversely, the "not why people buy from you" list:
- because you hope people will buy from you
- because it would make your parents proud of you
- it would make your investors would yell at you less
You should choose a reason from the first list, and make it a compelling reason, and make sure you are the compelling answer to that issue. Sometimes people buy from you because you got lucky. But that is an incident, not a business model.