Because the results of (well constructed) lotteries are random, no one ticket is any more likely to win than any another. 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.”
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.