I think it's important to realize that we fantasize the person more than actually knowing them in the beginning. This can be bad if you're expectations are high and you find yourself fantasizing about a partner often. It's especially hard when just chatting online as we tend to form that person in our head before we actually meet them.

Just remember it takes time to really get to know someone. You can spend a lifetime learning someones inner workings and never really figure it out 100%. It can be a blessing though since it will allow you to fall in love with the person more than once.

edit: meant to add that it's also a similar struggle if you fantasize about "The One that Got Away". It's not reality, that person isn't who you believe them to be in your dreams (both literally and figuratively if you dream about them at night). Your perceived notions of what that relationship would be might hold you back from actually fully committing to the person you're interested in.