Cain offered a sacrifice from that which was cursed (Genesis 3:17, "...cursed is the ground..."). It can be assumed that Adam and Eve passed that understanding along to both Cain and Abel and that both of them were fully aware of what the proper offering should be. It was also the BLOOD that had a spiritual significance to God (sacred, it "cried out from the ground", Genesis 4:11) and it was the BLOOD offering that made all the difference in Abel's offering. So both Cain and Abel had equal knowledge of how to "please" The Lord (which should be our real objective today, instead of simply "obeying" COMMANDS), but Cain thought his offering was a better idea because of ego and decided to disobey. Cain had more regard for his own idea (just like man's partisan ways of religious institutions) than PLEASING The Lord.
The parent-child kind of love is that the parent will always love the child (in healthy families) regardless of whether the child REMAINS worthy of being favored by the parent. In romantic or friend relationships, this is not NECESSARILY the same, often the love is fragile and will only last as long as the other person remains worthy. God's love is a "parent-child" love (the human race is/are His children). Brothers and sisters should also share a family "sentiment" towards each other and this is the GENUINE sentiment that God wants us to have towards each other, but He also wants us to trust that we will always remain in His favor and His love as long as we let Him love us and we don't outright reject Him. Just as children (most often, even in an abusive family) will be concerned with PLEASING the parent (rather than simply obeying COMMANDS), we should also be more concerned with pleasing The Lord and sharing a SENTIMENT between each other and God.
There are always two aspects of explaining or understanding anything, the THEORETICAL and the PRACTICAL. In theory, Christianity (true Christianity) is desiring to be "pleasing" to The Lord and sharing the "sentiment" of love with our fellow human being (instead of just our brothers and sisters who have accepted Jesus as savior, "But if you show partiality, you commit sin..." James 2:9). In PRACTICE, Christianity is taught as a religion and the simple obedience of COMMANDS are stressed to be sufficient as a devotion, but we are supposed to let The Lord's grace be sufficient for us as Paul did and want a PERSONAL relationship with The Lord based on having the spirit of God (heart of God, resembling what was taught in The Sermon on The Mount/'beautiful attitudes').
It is unimaginable that anyone on earth would complain about "theoretical Christianity" (I'm naive). All throughout history, unbeleivers have rightfully complained about PRACTICAL Christianity because this distinction is not made. Christianity is taught as though we should be accountable to the institutional church instead of accountable to conscience first so that we can stand before The Lord DAILY with a clear conscience (Romans 9:1, Hebrews 9:14).