The young person who is looking for a spouse and thinks he can choose to love any one who is convenient and available is a blatant fool, but the married person who doesn't choose to love his spouse is downright wicked. The things are entirely different. Single people should wait til they fall in love to marry. This will make the business of choosing to love your spouse a pleasure and give you a head start in performing all the duties of married life.
The man or woman who is already married has no business falling at all. You have already made your choice. Hopefully it was a suitable choice and you fell in love with the person you married, but regardless of the past, it is now your business to put up whatever walls are needed to keep from falling any which direction. You have to choose to love the one you have chosen.
If a married man or woman falls inwardly, you still have a moral obligation to choose to love your spouse. You may find that you do not have the power to choose to love in the same way and to the same degree as someone who unintentionally falls in love. Love is like that. Emotions are like that. People are like that. We can't always choose what we feel. But we can choose what we do, and a married person has an obligation to be faithful to his family no matter what he feels. This is love. It is not the wild, emotional kind of love that makes falling in love fun. It is the sober, faithful kind of love, that makes being married safe. It includes the commitment not only to give the bare outward duties of love but to nourish every proper emotion and to starve everything that goes against it. In this sense love is a choice.
Young people starting out and planning to marry ought to have this kind of commitment, but you don't want that to be all you have. You want your marriage to start with all the warmth of emotion that makes the journey of life delightful. A person can't choose to feel that with just anyone. Love of this kind is something that happens to you. It isn't chosen. It can be nourished or starved depending on the requirements of the circumstances, but it cannot be forced.