Love as illness.
Understanding is not a cure, but this defines the apparent problem (the problem of love as illness). We are ill with love in that we don't or cannot understand how we are experiencing the strong overpowering love we do feel towards an other. This still relates to the passive idea of love, however, in this mode our passivity means we are under its thrall. Love controls us and is therefore an illness in that we normally control our lives in a direct rational manner. However, we might also disagree with this description absolutely. The activity of reason, loves dulling or numbing of the action. Reason as controller.
Most emotions/feelings can be understood and expresed as an individualised perspective, however, love that is mutually expressed, shared and recipricated is therefore something quite other going on. In this sense it is the most philosophically important expression of an emotion, hence the image of a test, if this is what Wittgenstein meant. Perhaps we could think of the individual's love (non-reciprical for another also as a test, if so, the fuller kind (shared) is then a public inquiry.