What each sees in the other
To an INFP, the ISTJ can feel like solid ground: someone who does what they say, keeps the household running and never forgets the car insurance. To an ISTJ, the INFP can feel like an open window: imaginative, morally serious and able to see what a situation means rather than only what it is. Both prefer a quiet night to a crowded room, and both take their commitments personally.
The holiday that has always been done this way
The ISTJ’s family has held the same holiday dinner, same menu, same seating, for twenty years. The ISTJ finds comfort in it. The INFP, who goes by personal values, suggests skipping the turkey this year, or volunteering somewhere instead. The ISTJ hears a rejection of something that matters. The INFP hears that their ideals don’t count.
Money and plans spark similar standoffs. The ISTJ budgets carefully and sticks to it; the INFP might give a generous sum to a cause they believe in without checking the spreadsheet. Communication adds a layer: ISTJs tend to speak in facts, INFPs in metaphor and feeling, and each can leave a conversation convinced the other missed the point.
INFPs need emotional intimacy, and ISTJs often show care through reliability rather than words. Without some effort on both sides the INFP can feel lonely in a stable relationship, and the ISTJ unappreciated in a loving one.
Fixed zones and open zones
Agree together which areas of life are fixed and which are open. Maybe the budget and the holiday dinner stay as they are, while weekends and decorating are the INFP’s to play with. That gives the ISTJ stable ground and the INFP real room.
When the INFP proposes a change, the ISTJ can ask, “What’s the principle here?” before judging the plan. When the ISTJ resists, the INFP can ask what the tradition protects. Translating each other’s reasons, again and again, is the core skill in this couple, and both tend to come out of it broader people.