I only do it out of habit.

我这么做只是出于习惯。

Many people add salt to their food out of habit, without even tasting it first.

许多人出于习惯往他们的食物中加盐，甚至都不先品尝一下。

We say, "I love you" to our parents or significant other when we hang up the phone, not because we mean it, but out of habit.

我们会在挂电话时和父母或者某个重要的人说“我爱你”，不是因为我们真的想这么说，而是因为习惯。

We stayed together out of habit.

我们出于习惯待在一起。

Or was it just out of habit?

那或许只是习惯。