The Shy trait in particular isn't any harder to learn than any other trait. Normally, the Get to Know interaction shows up until the two Sims become friends. You can still learn traits by just using Chat. Another way is to have them be mean to each other until the friendship goes back to acquaintance. Get to Know will show up again at that point.
Now, having said all this... I've noticed that since the most recent patch, the Get to Know interaction seems to be more random. I've seen is disappear during the acquaintance period and reappear during friendship.
Your two Sims don't have to know all of each other's traits to be happy together. Learning traits is mostly so you, they player, will know what traits the other Sim has so you can decide what kind of interactions will give you ++ in the relationship building.
One last thing for slpc04. I had to edit your posts because of incorrect grammar and spelling. Please read the Forum Rules (in my signature below) and use apostrophes and correct spelling when posting on our Forum. Thanks very much.