So basically to sum my theory up.
Tia/Calypso wants Jack to be her equal..to be sealed with her in immortality..firstly he must become someone who wants to love someone else and be with them on a long term basis..and he also has to be her champion or he will say no to her.
Tia is the one who is making Jack a champion by introducing Liz into his life..notice that as soon as Liz comes into his life..he completely transforms into a hero for her. She gives him a huge amount of bravery..she is his rock in life, the driving force that sends him home to her...She is the very reason why Jack the man is transforming into a champion.
I think that Tia and Beckett have made a deal before any of the events in DMC played out
his line:
Jack must find his place in the new world or perish. Not unlike you, Mister Turner. You and
your fiancée face the hangman's noose." - Beckett
and
"wedding stopped or fate intervening"
all add to this whole notion that HE has been speaking with Tia before hand and has found something out about Will/Liz that could possibly threaten him and the EITC in the future?? Remember Liz is in a high state of power and Will/Liz being married/together might become a threat to them and their progress in the future..
so he says..they face the hang mansnoose if they stick together..and its all because of him.
So beckett finds something..but what about Tia's end of the bargain?? She wants JACK to be her champion and the only way for that to happen is if she gets LIZ in his life to make that happen..Also if Jack becomes immortal..it means that Jack is dead basically..its death..he will never be able to walk on earth again..so Beckett wins out ay??
Beckett legally stops the wedding..hes the only one with the power to...and he gives them umpteen hints that he is actually doing them a "favor"
anyway Tia thinks that Jack will probably leave Liz in a heartbeat when she offers Jack immortality...but she is seriously underestimating Liz and how Jack feels about her...and so is everyone else in the story..and so is the audience
In the Odyssey Calypso thought she was making Odysseus want her more..by making him equal to her..but all she was doing was making him love Penelope all the more..which is whats happening with Jack/Liz IMO as things progress between the two in the movies..