OP should read The Four Loves by CS Lewis. Unconditional love seems irrational if love is mainly a kind of feeling, but that's not the highest kind of love.
At bottom, what is love? It is a rational orientation of the will toward a good (you cannot love what you do not know). We can, therefore, desire another for our own good, and we can will the good of another for their sake. The first is eros, the second agape (it also falls under charity or benevolence). Different human relations involve different mixtures and manifestations of these two basic loves. Both are good.
However, agape is the selfless form of love. This is giving love and something we can choose and decide to have. It is not subject to the volatility of emotion. Indeed, when we say that perfect love (benevolence) casts out all fear, it is precisely because we have placed the good of another above our own desires.
Apage is the basis for successful marriages, and all good friendships, in general. This is why marriages of convenience are often more successful than those that begin in romance. This isn't because the latter is bad or that romance is opposed to having a good marriage. Not at all. Romance is good. It is only that very often the expectations of the latter are more likely to be rooted in emotion, while the former has no such expectation. The latter is more prone to confusing love with romance. Love involves suffering and sacrifice, and so those who view love in terms of pleasant emotions are in for a world of disappointment.
The Pali word 'mettā' means something quite similar to 'agape'. In Buddhist traditions, mettā (or loving-kindess) meditation is all about cultivating this sort of sentiment by conscious effort. From the very little I've practiced this, I can confirm your point about how benevolence, for some reason, reduces fear and anxiety (often we aren't even aware that it's there).