Before his first luncheon with Sebastian (a peace offering after the puking incident), Charles pauses to consider whether or not he should go. He was uncertain, he says, "for it was foreign ground and there was a tiny, priggish, warning voice in my ear which […] told me it was seemly to hold back." But look at his eventual reasoning for attending:
But I was in search of love in those days, and I went full of curiosity and the faint, unrecognized apprehension that here, at last, I should find that low door in the wall, which others, I knew, had found before me, which opened on an enclosed and enchanted garden, which was somewhere, not overlooked by any window, in the heart of that grey city.
Enchanted garden…sounds a bit like "Arcadia," doesn’t it? Charles certainly gets what he was looking for when he dives headfirst into this friendship with Sebastian. As we talk about in "Character Analysis," Sebastian truly does open up a whole new world for Charles – a world of youth, care-free days, wine, and, most importantly, of art and beauty.
But much later, when Sebastian has become an alcoholic and Lady Marchmain is angry with Charles for supplying him booze, Charles drives away from Brideshead for what he thinks will be the last time and remarks, "A door had shut, the low door in the wall I had sought and found in Oxford; open it now and I should find no enchanted garden." This is more of that paradise/death stuff we’ve talked so much about in regards to the phrase "Et in Arcadia Ego." That ever-present skull tainting the perfect pastoral landscape has brought to an end those "heavenly days at Brideshead."