Humans are stubborn. Anyone who has lived any amount of time can tell you that if someone makes up their mind it takes a gargantuan effort to make a dent in someone's convictions. Add to that we don't listen well. Quite frequently we hear what we want to hear, and anything else is lost as trivial babble...no matter how important it is.
Perhaps that is why Jesus speaks of love so frequently, that St. Paul wrote his famous chapter in his first letter to the Corinthian Church (1 Cor. 13), that St. John in his first letter to the Church Universal described it so completely (1 Jn. passim). Today's Gospel fits the same mould (Jn. 15.9-17) as Jesus tells His disciples...again...still...to love one another. Even the Old Testament, which many see as stark, harsh, and unforgiving, speaks frequently about love.
First, love is the most difficult choice we can make. We can adopt rigorous spiritual practices, be upstanding and moral people, adhere to all laws and rules, but we do have a terrible time loving our neighbour as ourselves. Many of us even have a tough time loving ourselves; not that selfish indulgent self-interest that gets called "self-love", no, that is not what we are talking about, but that loving yourself where we strive for what is best for us even in the face of discomfort, where the focus is not on self but on becoming a source of love one's self...THAT is difficult.
Second, we so frequently misunderstand what love is. That is the fault, I suppose of language, where many languages use their word for love to describe a purely emotional state, where warmth and favour are bestowed on the object of that love...but favour is equally applicable to your spouse as to one's favourite snack, and that is not real love, the love Our Lord commands us,
Third, love is often counter to self interest. If we truly loved someone we would do what we could for them, the consequences to ourselves being insignificant. In a perfect world, while we are looking out for others, others are looking out for us, but sadly, the world is imperfect, which is why Our Lord warns us that the path of love is difficult.
This enjoinder to love is frequent in Scripture because it is so difficult, because we in our isolation and inward focus find it counter-intuitive. It is the choice our fallen nature kicks against. It is the very stuff of God, however, for God's love is selfless and for all, for as St. John reminds us, He is Love, and in His love He wants us to be like Him, and to be like Him is to love like Him, no matter what. It bears repeating, and God repeats it often, if we but listen.