Concerning love as it hits Viola (Cesario), Olivia and Orsino (Duke), in the simplest terms, they all suffer from unrequited love (unreturned and unfulfilled love). This trio of unrequited lovers forms a triangle of love: Viola loves Orsino who loves Olivia who loves Cesario, Viola's male-disguised persona. The viewer (reader) of the play is prompted to wonder which one of these is the most foolish in their feelings of love.
On another level, Shakespeare is presenting the painful side of love. Each lover uses expressions denoting pain and suffering: plague, cruel, desperate. An underlying suggestion is that their personal choices have brought this on each lover, but they themselves insist that their suffering is an affliction bestowed on them against their will, as it were. It is up to the viewer/reader to entertain these two separate notions and decide whether the ills of love are self-inflicted or inflicted by Fate.
The element of disguise can't be overlooked in considering the loves of Viola, Olivia and Orsino. In the simplest terms, Viola's male disguise opens, for some, the question of the shallowness of love's attraction but, for others, it opens the question of what traits are actually attractive and worthy of love.
To elaborate, if Olivia is not a superficial, shallow, unobservant specter gliding mindlessly through life, then her love for Cesario/Viola can be presumed to be built on traits that are appealing. Because Cesario/Viola is really a woman, some can conclude that Shakespeare is suggesting that it is Viola's traits of kindness, gentleness, rational conversation and personableness that attract Olivia, who is then the Shakespearean represetative of universal woman.
[It is interesting to note that this resonates with the contemporary question of women's seeming attraction to "dangerous" men who are cold and unloving.]
For more information read Lee Lady at Hawaii.edu.