In Twelfth Night, Shakespeare uses the 16th century Festival of Twelfth Night to demonstrate the strange association of love to madness. Nowhere is this more pronounced than in the "notoriously abused" situation Malvolio is caught in. Fabian and Toby, with Maria and Feste, aggravate Malvolio's secret and over-reaching feelings of love for Olivia and egg him on to behavior that makes him look like he has become "distracted" (mad), then lock him up in prison thereby proving his madness while simultaneously attempting to convince him of his madness.
Orsino opens the play by drawing an analogy between music and love, with the hope that an excess of music might satiate the appetite of love and sicken it so it might die. In the same scene (1.1) Orsino receives a message saying the Olivia will not allow Orsino to woo her because she will spend seven years in morning, weeping each day for her dead brother. Both these situations render love as an excess that compels the bearer of love to irrational behavior. Viola comes along and sheds some wisdom on a sounder, more stable form of love, though it too is a love borne at first sight.
Love in the Twelfth Night appears in many forms.
There is for example sibling love between Olivia and Sebastian that proves to be honest and true. On the other hand, we have parody of petrarchian love in Orsino passion for Olivia whom he doesn't even now well. Orsino is a bit comic moaning all the time. Accordingly to petrarchian conventions he vividly describes his pain:
Olivia's love for Orsino is of very loyal kind. she only wishes him to be happy and is even ready to sacrifice her own happiness for it. on the other hand, he falls in love with her suddenly and we cannot be sure of the depth of his emotions.
Similarly, Sebastian's love for Olivia is love at first sight. But is marrying a women you don't know really wise?
Olivia seems to have loved Viola passionately when she thought her a man. Furthermore, her love was not based only on appearance but on Viola's character. However, it is not clear whether love between Olivia and Sebastian will be of the same quality. After all Olivia only married him because she thought he was Viola ( that is a man that Viola pretends to be).
Also, we have a true love that is certainly homoerotic if not homosexual in case of Antonio who is completely attracted to Sebastian and expresses his feelings freely.
Malvilio love for Olivia is false and is based only on ambition and desire for power.
Finally, there is a love between Maria and Sir Toby that is realised in the end despite the class differences. How far is this love true it is hard to know for sure.