Because that's just how humans actually are you know. Despite both of them showing just how much they love each other, you always have that thought somewhere deep in your mind, if you are good enough for the other person.
Personally for me, it's actually kinda refreshing to see. It's not like i like seeing them both fight, but it's just show a realistic situation that happens between couples, or even between families and friends. There's always ups and downs in a relationship.. but i do hope the next chapter they make up with each other (and make out

I don’t understand why they’re both so insecure… like, they love each other and they have rings n stuff, what are they so afraid of?