Affirmations are intended to shift the way you think. But repeating "I AM happy" (stated as a fact occurring in the present tense) when you're sad or depressed, can be like beating your head against reality, because in that moment you're saying something you believe to be untrue! That feeling of conflict keeps you separate from what you seek. The Law of Attraction works on the feeling.
When you say "I LOVE BEING happy", it remains true whether or not you are happy in the moment. Since you DO love being happy (unless you are irretrievably masochistic!) there is no conflict about the truth of the statement. It feels safe to keep saying it, and that helps you build an emotional bridge between where you are and where you want to be. The "I Love..." affirmations are always phrased as a process that is happening in the eternal now.
The affirmations are collected in subjects such as relationships, calm, release, assertiveness, rather like mini-workshops on each topic. As you will see, each subject is matched with an attractive and relevant photograph. The combination gently and simply shift your thoughts in a more useful direction.
The Law of Attraction page at the back of the book has suggestions for you to create your own affirmations relating to your own personal goals and desires.