Dear Uglytruth, I think it's best to watch out for too much literalism in your interpretations. I think what John is trying to say here is that being too much in love with a carnal world is not the way to go. I don't think he was saying that it is wrong to have a satisfying, enjoyable, loving life. Life of the Spirit is knowing how to detach from the lures of the physical world. We have a good example -- Jesus.
I think Living_in_the_Light has introduced a relevant and worthwhile correction, up to a point.
I am reminded of a ditty my mother used to recite to me when I was young: "Quaker meeting has begun. No more laughing, no more fun." And Puritans still have a reputation of shunning pleasure even if the reputation was not entirely earned.
I don't mean to pick on Quakers or Puritans (I grew up in once-Puritan Massachusetts not far from once-Quaker Pennsylvania), but to suggest that the idea has been around for a while that being a Christian requires asceticism--self-denial of pleasure and fun.
Of course there is a kind of half-truth in frowning on pleasure and fun, namely that the sins of the flesh (meaning sin nature) usually occur in intimate relationship with the flesh (meaning human body)--e.g., self-centered anger (feel the heat?) and sexual immorality. We usually sin with the body.
But the human body itself is good. God created it. The physical world is good. God created it and called it good (if now also cursed). And God as a good God made many pleasures in this world for our enjoyment and His glory.
For the Christian to die to self, to hate the self in the John 12:25 sense, may indeed mean denying oneself a certain pleasure or fun, but not because enjoying many pleasures is necessarily wrong.
Rather, our sin nature misuses and abuses the thing from which one might derive pleasure. Then there are human pleasures which are so twisted as to be evil themselves, such as enjoying another's suffering--but I assume that is not the "garden variety" sort of pleasure.
More commonly I think, we can become so caught up in pleasures and fun--laughing with friends, watching an engaging media program, practicing in a hobby, listening to music, whatever--that we neglect duty or the will of God. Or we can use pleasure to indulge the sinful nature (e.g., gluttony).
For example--an perhaps unexpectedly--we can become ascetics (deniers of one's own pleasure) in order to feel pride at being spiritual or righteous. Take the Pharisees in the Gospels (who fasted two days a week, for example).
And Paul tells the church at Colossae that there are religious practices which have an appearance of asceticism and wisdom, but they "are of no value in stopping the indulgence of the flesh [or sinful nature]" (2:23).
Therefore for the Christian "hating one's life in this world" (again John 12:25) does not mean denying all sense of pleasure, though it may mean curbing pleasure where duty demands. Such denial of the self and "hating self," however, does mean dying with Christ to that self which is contrary to God while living by faith through Christ in righteousness (I think of Galatians 2:20 for example, or again Romans 6).
And living by faith through Christ can mean enjoying certain pleasures (sex with a spouse, tasty food, music, socializing, a cooling fan or warming furnace, and so on) and thanking God for it. And have you ever enjoyed God Himself?
Using the body and God's physical world as God wants us to may (will) involve both pain and pleasure. Loving God and one's neighbor is key throughout. Those who have eternal life are on a path of holiness throughout because God in Christ is conforming one into His image, particularly as one obeys Christ.
