Doubt in a long-term relationship?

For the most part, my SO and I are solid about our relationship and our plans to move in, get married, have kids, settle down somewhere, etc. However, on occasion, when we have terrible arguments, my SO will say things like he isn't sure if he's happy in this relationship, or he isn't sure things will work out in the future, or he doesn't know if he still loves me, or he thinks we need a break, or he'll say that I need to work on myself. More often than not, he is the one saying these things while I tend not to. Is this normal or a red flag?