When the app first came out (or possibly when you first start using it), the first person is/was always a match. Then they got clever about hiding matches deeper in the swipes.

This triggers addictive behaviour because the "match" reward is inconsistent. Though, clearly, before you start swiping, Tinder knows which matches it is going to present to you. It's a delicate balance to judge how long you will spend swiping, when to show you your match and how to make sure you swipe right a few times to keep the funnel loaded.