I'm don't mean you got to be serious all the time. Definitely alittle humour and playful at times will really spice up the relationship. Be serious FIRST when it needs to be, then when the matter lighten up, joking about it is fine.
When talking about big matters, be serious first. When she's feeling really down or when she lost something that really meant very important to her? Be serious first. When she's jealous, angry, sad, worried, be serious first. Sometimes we just tend to take things lightly because both of us are together for so long. I experience gers that suddenly ask you a question and you thought its just another casual talk of hers? You reply her in a joking manner. What is bad when it turned out to be some kinda test/decision of hers and she's really upset by how lightly you take her and her question. She may feel insecure and may find another person who treats her seriously. Sometime girls are just so hard to understand.
I remembered once when i got angry with my gf for not seeing that i'm unhappy about something. Instead, she scolded me because she don't know whether i'm serious or i'm joking.
