The conversation between Duke Orsino and Viola in Act 2, Scene 4 is less a conversation about how men fall in love and more a conversation about how deeply men fall in love. The recurring question in the speeches is, who loves more deeply, women or men?
Even Orsino admits that men have fickle natures, as we see him confess when he advices Viola, whom he thinks is Cesario, to fall in love with a woman who is younger than he is. He confesses that men's "fancies are more giddy and unfirm" and even waver sooner than women's fancies, meaning that men are far more fickle than women (II.iv.37). Viola certainly agrees with this statement; therefore, one way in which men love is through changing, fickle notions. He even admits that women lose their beauty once their beauty is in full bloom, like a rose, and it is this loss of beauty that can make a man fickle. However, once stating that men are more fickle than women, he later argues that no woman could love him as deeply as he loves Olivia.
In contrast, Viola, being a woman, argues that she knows "[t]oo well what love women to men may owe," meaning that she knows very well just how deeply women can love (113). She even advices him to give up pursuing Olivia because there is a woman who loves him just as deeply as he thinks he loves Olivia. Therefore, all in all, what's being said here about the nature of men's love, or how men love, is that men are far more fickle than women and that a women's love is far greater and deeper than a man's.