진지한 봐 볼 때 항상 웃고있어 is not correct. 진지한 봐 볼 떄 should be 네가 진지하게 보일 때 or 네가 심각하게 보이면. 웃고 있어 means "I am laughing", but there are better ways to describe ending up laughing even though it's not your intention. You can say it like these. - 네가 심각한 얼굴을 하면 난 (항상 / 자꾸) 웃게 돼. - 네가 심각한 표정을 지으면/지을 때 난 (항상 / 자꾸) 웃음이 나와. - 네가 진지해 보일 때 난 (항상 / 자꾸) 웃음이 나와. (similar to your sentence) (They all mean "I always can't help smiling when you look serious / make a serious face.") * 심각한 = serious. 진지한 = earnest, serious. * -(은)ㄴ 얼굴을 하다 = make a certain kind of face, such as 웃는 얼굴(smiling face), 화난 얼굴(angry face). * 표정을 짓다/하다 = similar, but emphasizes the facial expression (표정). 웃는 표정을 짓다/하다, 화난 표정을 짓다. * 자꾸 = repeatedly (despite one's effort not to do it). * 웃게 돼 = I end up laughing (even though I don't want to). Something makes me laugh. * 웃음이 나와 = (literally) laughter bursts out. Can't help laughing.

Maybe: "I always smile when you look into my eyes." Hard to understand your intent or what the intent of the other is. The word "seriously" may mean that the person is looking more intently than a simple glance. Sounds more "serious" like looking at you to see your expression or understand your heart of kindness through your eyes (looking into your eyes is serious).