Final answer:
The investment model best explains why this relationship is likely to remain stable.
Step-by-step explanation:
The theory that best explains why this relationship is likely to remain stable is the investment model. According to the investment model, commitment in a relationship is influenced by three factors: satisfaction, quality of alternatives, and investments made into the relationship. In this case, the couple has already made sacrifices for each other, which can be seen as investments, increasing their commitment to the relationship and making it more likely to remain stable.
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