Jack and Wanda have known each other longer than most other units within Gobwin Knob. Both Wanda and Jack come from the Kingdom of Faq and turned to Stanley's side when he destroyed their native kingdom. While in Faq Jack and Wanda used to spend time together discussing the philosophy of magic.
One philosophical question that fascinates both Jack and Wanda is the nature of Love. Is Love a form of Natural Magic, perhaps a form of Natural Thinkamancy or Natural Hippiemancy? Jack believes that Love is a force which is far more powerful and mysterious than any form of magic. As he once told Wanda, "I suspect, love is something greater than magic itself, perhaps something even the Titans are helpless before."
Wanda and Jack's interest in this question was probably related to the fact that they were both deeply attracted to Jillian. Jack doesn't seem to have been angry or bitter that Jillian favored Wanda over him. Wanda made a point while in Faq of using Jack's full name and title, suggesting that she wished to maintain a certain degree of formality in her dealings with him.
Jack currently thinks that Wanda is a "vessel cracked worse (and repaired worse) than he," referring to Wanda's emotional and mental state, and may resent the fact that Wanda suggested allowing him to croak so that she could decrypt him. However, any concern he may have about her growing power didn't stop him from having some fun at Wanda's expense by pointing out that Jillian may now prefer Ansom over both Wanda and himself.
Jack has also implied that he may be aware of some of the details of Wanda's role in the fall of Faq, specifically what went on at the battle for the capital when the King was croaked.