How can the prick of a needle feel so little and mean so much?

I went to get my final hepatitis A & B vaccine today, after getting plenty of those last summer. Now I won't catch hep A for 25 years. As for hep B...

Nurse: You won't be able to get infected ever.
*smiles*
Me: Great.
Nurse: Hopefully.
*turns and walks away*
Me: Great...