Because the results of (well constructed) lotteries are essentially randomplugin-autotooltip__plain plugin-autotooltip_bigRandom numbers

unknowable

"We can never decide for sure that a number is random, but what we can do is apply an increasing number of tests and treat our sequence of numbers as innocent until proved guilty."

Source : Prof. Colva Roney-Dougal, senior lecturer in Pure Mathematics at the University of St Andrews, speaking in, no one ticket is any more likely to win than any other. However, several studies have shown that those who are in possession of a lottery ticket are usually very reluctant to swap it (even though the other ticket has exactly the same chances of winning).

A 2007 study performed at Cornell University suggested that :

[..] this effect occurs because the act of imagining an exchanged ticket winning the lottery increases the belief that such an event is likely to occur."

[source link below]

Another theory is that gamblers are protecting themselves against the feelings of regret they might experience if, after having swapped a ticket, their original ticket ended up winning.

See:Another Look at Why People Are Reluctant to Exchange Lottery Tickets Journal of Personality and Social Psychology, Vol. 93, No. 1, 12–22

Also see : The Envelope Paradoxplugin-autotooltip__plain plugin-autotooltip_bigThe Envelope Paradox

The Envelope Paradox is a mathematical / philosophical puzzle, variations of which date back to (at least) the mid 1950s.

"You are given two indistinguishable envelopes, each containing money. One contains twice as much as the other. You may pick one envelope and keep the money it contains. Having chosen an envelope at will, but before inspecting it, you are given the chance to switch envelopes. Should you switch?