Alcohol even presents similar issues. If you're in a bad mood, drinking can make you angry and violent. If you're in a good mood it can make you relax. There's definitely a more amplified effect in the case of acid, but it's the same principle.
As Humphry Osmond famously said, "To fathom Hell or soar angelic, just take a pinch of psychedelic." The problem is, you don't know which you'll be doing in a given trip, and it's not uncommon to do both in the same trip, even bouncing from one to the other many times. There are many, many factors that play in to this, some of which are mentioned elsewhere in this thread.
Now, take any 4 weekends from the past six months and compare them, what was on your mind for each Saturday and Sunday? Count the things you might think about in a span of time for any day off.
Now, imagine that thrown into the middle of that, your friend shows you, in the palm of their hand, two tabs of acid.
Add psychedelic state to all your shifts in mood, stream of consciousness and common distractions, and dredge up the baggage of memories reaching back into your childhood, and include existential questions about what it means to exist.