Happy thoughts make me happy, but--whoop!--how quickly I swing again into obsessive worry, blowing the mood; and then it's the remembrance of an angry moment and I start to get hot and pissed off all over again; and then my mind decides it might be a good time to start feeling sorry for itself, and loneliness follows promptly. You are, after all, what you think. Your emotions are the slaves to your thoughts, and you are the slave to your emotions.
The other problem with all this swinging through the vines of thought is that you are never where you are. You are always digging in the past or poking at the future, but rarely do you rest in this moment. It's something like the habit of my dear friend Susan, who--whenever she sees a beautiful place--exclaims in near panic, "It's so beautiful here! I want to come back here someday!" and it takes all of my persuasive powers to try to convince her that she is already here. If you're looking for union with the divine, this kind of forward/backward whirling is a problem. There's a reason they call God a presence-- because God is right here, right now. In the present is the only place to find Him, and now is the only time.
But to stay in the present moment requires dedicated one-pointed focus. Different meditation techniques teach one-pointedness in different ways--for instance, by focusing your eyes on a single point of light, or by observing the rise and fall of your breath. My Guru teaches meditation with the help of a mantra, sacred words or syllables to be repeated in a focused manner. Mantra has a dual function.
For one thing, it gives the mind something to do. It's as if you've given the monkey a pile of 10,000 buttons and said, "Move these buttons, one at time, into a new pile." This is a considerably easier task for the monkey than if you just plopped him in a corner and asked him not to move. The other purpose of mantra is to transport you to another state, rowboatlike, through the choppy waves of the mind. Whenever your attention gets pulled into a cross-current of thought, just return to the mantra, climb back into the boat and keep going. The great Sanskrit mantras are said to contain unimaginable powers, the ability to row you, if you can stay with one, all the way to the shorelines of divinity.
Among my many, many problems with meditation is that the mantra I have been given-- Om Namah Shivaya--doesn't sit comfortably in my head. I love the sound of it and I love the meaning of it, but it does not glide me into meditation. It never has, not in the two years I've been practicing this Yoga. When I try to repeat Om Namah Shivaya in my head, it actually gets stuck in my throat, making my chest clench tightly, making me nervous.