There are many possible reasons why Leia might be reticent.
First, her feelings for Han may not have fully crystallized. Her relationship with Luke has been, well, very friendly. From Episode V:
INT. MILLENNIUM FALCON - SLEEPING QUARTERS
Luke rests on a cot, his injured arm wrapped in a protective cuff.
Leia gently wipes his face. The ship lurches again.
LEIA I'll be back.
She kisses him, then leaves the quarters.
But this is unlikely. Leia is a straight-shooter, and she has had a sisterly rapport with Luke (consider their first encounter on Episode IV).
So I favor a much better alternative. Being a Jedi is far from trivial. As Luke's experience shows, it requires more than fine-tuning telepathic skills to properly wield this power. And others can feel when you wield the force. Darth Vader and Luke have felt each others presence. Luke tells Leia of this:
LEIA Luke, tell me. What's troubling you?
LUKE Vader is here...now, on this moon.
LEIA (alarmed) How do you know?
LUKE I felt his presence. He's come for me. He can feel when I'm near.
That's why I have to go. (facing her) As long as I stay, I'm
endangering the group and our mission here. (beat) I have to face him.
Leia is distraught, confused.
But Vader has not felt Leia's presence:
EMPEROR I told you to remain on the command ship.
VADER A small Rebel force has penetrated the shield and landed on
EMPEROR (no surprise) Yes, I know.
VADER (after a beat) My son is with them.
EMPEROR (very cool) Are you sure?
VADER I have felt him, my Master.
What's more, Luke explains to Leia her importance:
LUKE There's more. It won't be easy for you to hear it, but you must.
If I don't make it back, you're the only hope for the Alliance.
Leia is very disturbed by this. She moves away, as if to deny it.
LEIA Luke, don't talk that way. You have a power I--I don't understand
and could never have.
LUKE You're wrong, Leia. You have that power too. In time you'll learn
to use it as I have. The Force is strong in my family. My father has
it...I have it...and...my sister has it.
Leia is unsure of the effect this information could have on her crew, including Han. It is no small feat to manage a rebellion being doggedly hunted down by the ruthless Empire. But there might be a better reason to keep quiet: she might become their last hope, and will have to keep hidden from the Empire while learning how to wield her Jedi powers.