Don't even try to lighten things up between you, because it doesn't work! You're serious and intense when you get together. You actually both really appreciate the effect you have on each other.
Whereas other relationships in your lives might be more superficial, allowing time only for small talk, this one is more intense and focused. You discuss all the deeper, more complex and weighty issues that you both might wonder about -- the intricacies of your relationship; your pasts; sex, death, psychology... You help this person deepen in the ways that they think and communicate, and they help you in turn in accessing and translating your deepest thoughts and feelings. As time goes on, you each probably find that you feel a greater sense of understanding for yourself and your place in the world. You understand each other better and better over time, too. You feel a strong pull whenever you hear each other's voice on the phone or see an email in your inbox, sent from your lover -- you know it's going to be about something interesting, something you want to know about. As long as you don't start looking to each other for undue validation for your ideas -- meaning, maintain your independence and appreciate the differences between you -- this is a very positive and transforming influence for your relationship.