Not all ISFJ's complain. Everyone complains, that has nothing to do with type. ISFJ's are actually more prone to hold things back as not to hurt others feelings and to keep harmony.
One of the biggest misconceptions about ISFJ's is that they always complain. It comes from a fundamental misunderstanding of what they are trying to communicate. A lot of times, ISFJ's will use a bad situation in there life in order to get someone to talk about what is going on in their life. We try to be subtle, and we do too good of a job about it. Its also how we phrase things. I myself am an optimist, but if you ask any of my NF friends, they say I am a pessimist. It is just how they perceive how I speak.