What is the cause of madness in love?
A lax or tepid interior life (that is, if there is any sort of interior life that approximates the life of prayer that is worthy of the ineffable thaumaturgy of supernatural grace) will lead an individual to excessive and abusive attachment to self and other created things, in a vain attempt to fulfill the happiness necessitated by the unfathomable immensity and profundity of the human soul: God alone, knowing, loving and serving Him, can make the human soul truly happy.
A lax or tepid interior life is one that is bereft of the magnanimity, generosity, and courage to practice mortification and self-abnegation as much as one can practice them (the duties of state, the counsel of Spiritual directors, the inspirations of grace, &c., may determine the sorts of mortification that an individual ought to practice), and the soul negotiates away the liberty of holy grace by giving in to the "enfranchisement" that the world and the flesh offer, which is really the tyranny of the devils.
Without God, the soul is driven to either distract itself from its appalling interior chaos with the things and thoughts of the world, or is compelled to make an idol out of something or someone which they believe will make them happy. That thing that the worldings call love is only a complex sequence of neuro-chemical processes that are manipulated for egocentric satisfaction: when those processes cannot be manipulated so that satisfaction is attained, then exterior factors and variables are manipulated to get the desired results.
With human nature and society so vitiated by original sin (worsened by the course of the ages and the exacerbation of ever-growing personal and public sins), a soul can gradually lapse into a psychopathic state wherein it debases itself even to sadism, however subtle it may be. Mortal sin itself in its awful plenitude is a sort of sadism, a relish of the hatred of God and of self, and a soul in such a state cannot truly love itself or anyone else.
The pseudo-civilization of the present age is designed so that if you are not cultivating the interior life worthy of Christian grace, then you can theoretically succuмb to the basest forms of pathology and commit the most horrendous acts, even whilst remaining blind or insensible to the gravity of these matters.
It's not a coincidence that serial killers and vicious criminals were never persons given over to prayer and meditation.