Firstly, I'm a female ISTJ. And this doesn't define every ISTJ, but for me being with others mostly makes me tired and at the same time I don't want to feel lonely. The point is wanting to be alone doesn't mean I want to feel lonely. And we always say, we're not robots. Of course we need romance in relationships, but for me everything has a time. Like, I want to get close when we're alone (so my family members cannot see us touching kissing etc, but hugging while we're both happy). I don't want to show my emotions even though it's my mom&dad, but my SO. Maybe some male ISTJ's don't show their emotions even though their SO. Of course it's hard for you to make a sense, ENFP's are extrovert, and you know what we're. And not every ISTJ's the same. So if you want an answer, ask your ISTJ's, it's the only way to really know.