You attract what you feel---not what you think.
If someone asks how you're doing, and you tell them you're doing great, but you're really feeling like crap, you will attract the vibration of how you're truly feeling--not what you're telling others.
Every single increment of your life is based on feelings. When we wake, we gauge our day by how we feel. Do we feel happy or depressed. Stressed or angry? Sad or blah? Our relationships are based on how we feel as well. Do they make us sad, glad, or mad? Our jobs? Are we feeling happy about going to work, or resentful? Every single thing we do produces a feeling, and it's this feeling that attracts events that are vibrationally equal to it. There isn't a moment in our lives where we don't gauge our feelings. And this is what the law of attraction is all about. It is based on how we feel.
Choosing to think positively about an idea is the first step in making a shift. In time, you can shift your beliefs. When you shift your beliefs, you begin to feel differently about the beliefs you've changed. It's a process; however, it's the way the physical plane operates.
We attract all situations and events based on how we're feeling at any given time. If we continually focus on how we feel over a particular topic, we will attract more of that topic--be it good or bad.
Take a day and examine how you feel. Go to work and examine how you feel about the commute, the people, the office. Go to the store and examine how you feel about the checkout clerks, the store itself, the people around you. Do the same for your family and love life. Think about it. Every single thing you do is based on how that thing makes you feel.