Basically, there's a belief that "7" is a lucky number. Mostly attributed from slot machines where 7-7-7 would mean a jackpot.
Although considered "lucky" 7 is actually the most common number when rolling two dice. This is because there are more ways to make 7 then there are any other number from 1-12 on a 1-6 dice.
seven can be made by:
1+6, 2+5, 3+4, 4+3, 5+2, and 6+1
Another reason is that it is "apparently" not a common number. For example, if you ask a person to say a number from one to 10, if he or she says 1, that's boring because it's the first number, if he/she picks 10, that's boring cause it's the last number, if he/she picks 5, that's boring cause it's the middle. So really, when people try to be individualistic, they have a higher chance of picking a number like seven, or three, just because they don't want to be boring.