When you're in one, you know why.
It's not the most helpful advice, but really there is absolutely no way to adequately explain it to someone who is incredibly cynical because it defies logic and reason. When you really love someone, even if it isn't a sexual relationship, you know absolutely, no questions asked.
Of course, some people get so cynical that they completely cut themselves off from the possibility of ever feeling the emotion. Think about it like in
The Magician's Nephew (sorry, just finished reading it so this is the best I've got). Uncle Andrew convinces himself so much that the animals aren't talking and that Aslan isn't effectively Jesus that he is completely unable to see past himself to witness the wonders etc of the new Narnia. That's someone who gets so bogged down in cynicism that they can't see past it, they miss out on some of the best things about life.
Of course, to temper this gushing a little, there's also the Equilibrium problem 'at the cost of the dizzying highs we suppress the abysmal lows.' If you accept that much opennes, you will get hurt, a lot. But it's up to you to decide if it's worth it.