The discourse particle ja is normally used to signal that the other person already knows what you say, that is, it's part of the common ground. As such, it is, in it's 'basic' use, roughly equivalent to as you know. As such, it can't be used in the following sentence
DONT: Ich muss dir unbedingt was sagen: Ich liebe dich ja! (lit. I have to tell you something: As you know, I love you!)
However, it can be used to backround information that you believe is known to the person you're talking to:
DO: Ich muss dir unbedingt was sagen: Ich liebe dich ja, und deswegen will ich dich als Erben eintragen. (lit. I have to tell you something: As you know, I love you, and so I decided to make you my heir)
Even in 'surprise contexts' it is necessary for the other person to already know the information, so, you can only express that you yourself are surprised by some information, but ja cannot express that it should be surprising to the other person that something is surprising. Again a nice example
DONT: [As a gynecologist you inspect a person and say] Sie sind ja schwanger! (You are pregnant, and that is something you already know but I find it surprising!)
DO: [As a person finding the positive pregnancy test of the person s_he is living with] Du bist ja schwanger! (You are pregnant, and I didnt know that before!)
Note that ja does not bear any positive or negative connotations. It's a very useful German word for structuring discourse, I'd definitely recommend using it. It let's you tell somebody something without sounding overly patronizing in case the person already knows this information.
This answer is based on the following thesis:
Bárány, A. (2009). Form and interpretation of the German discourse particles ja, doch and wohl. Diploma thesis. Available from: http://othes.univie.ac.at/7532.