Duke Orsino's views on love prove to be a bit contradictory, and all of his contradictory views culminate in his sudden, fickle switch from loving Olivia to loving Viola by the end of the play. However, one view of Orsino's that does remain constant is that love can be very cruel.
We see him expressing his view on love's cruelty in the very first scene. Here, he compares love to the deep, vast sea, saying that just like the sea swallows up things of value, like in shipwrecks, making those things completely worthless, love also swallows up things of value, rendering them worthless. In these lines, he is thinking of himself, his own value and merit, and thinking that all of what he is has now become completely worthless since falling in love with Olivia due to her rejection of him. Hence, love takes what's valuable and makes it worthless. In this scene, he further describes love as being cruel when he likens himself to a hart, or deer, being pursued by his cruel desires for Olivia. In other words, since his desires won't leave him alone, he sees himself as being hunted and tormented by his desires.
Orsino expresses contradictory views on love later when talking with Viola as Cesario in Act 2, Scene 4. In this scene, he begins by speaking of the constancy of true lovers and advising Cesario to love as Orsino does, if ever Cesario should fall in love, because Orsino is as constant as "all true lovers" in that he thinks of absolutely nothing but whom he loves, as we see in his lines:
For such as I am all true lovers are,
Unstaid and skittish in all motions else,
Save in the constant image of the creature
That is beloved. (II.iv.17-20)
More specifically, Orsino is saying here that he is "unstaid," meaning "changeable," and "skittis," meaning "fickle" in all things other than the one he loves (eNotes). However, even though he claims to be constant in these lines, he next contradicts himself by asserting that all men are fickle. When he learns that Cesario is in love with someone older, Orsino advises him to find someone younger because men's "fancies are more giddy and unfirm" and "sooner lost and worn, / Than women's [fancies] are," meaning that men's fancies are far more changeable and fickle than women's fancies are (36-38).
Therefore, while on the one hand he sees himself as being devoted to Olivia, he confesses that men are fickle, which is contradictory. His confession also shows his concept of love being a fickle emotion as well as a tormenting emotion.