Why Do I Keep Seeing My Birthday?

I assume you mean you keep seeing your birthday numbers rather than seeing the day itself in your imagination.

You keep seeing those numbers because those numbers have personal meaning to you.

You see other numbers, too. Lots of them.

But you generally only remember seeing numbers that have personal meaning — which your birthday numbers have.

Because you only remember seeing your birthday numbers, it seems like the only numbers you see are your birthday numbers.

Not everybody remembers it when they see their birthday numbers. Only people who, when they see the numbers, recognize that it is their birthday number and make a mental note of the fact.

See the Why You Start Seeing Certain Numbers Again and Again article and also the article Why People See Certain Numbers All the Time for more information.