Because Reality/Life is always and only in this Present moment, whenever the Present moment, also known as Now, is lost in the Past, Reality has been replaced by illusion. Because time is a concept of the mind, thoughts are what take you back to the Past where Ego lives.
Positive thoughts bring you back to good times and you're able to appreciate them Now for the blessings they are. Ego is selfish at best and savage at worst which explains why it is never satisfied with what is and insists the Future will be the same - unsatisfactory. Because Ego needs conflict and drama to maintain its existence, Gratitude is a repellent that evaporates Ego instantly!
To live in the state of Gratitude is to live Life Now. It is only in the Present moment that life is seen and valued for the priceless gift it Really is. Because Ego knows this, and you forgot this, it uses the Past to distract you and throw you off course. Fear is Ego's one and only trick, which comes in many different forms, to confuse you into believing because of the Past, there is little to no Hope which is why you need Ego to "protect" you. This is a sick and twisted way of thinking/thoughts because it makes absolutely no sense yet this seems to make perfect sense to those that have forgotten Truth.
Unless and until the Present moment, Now, is lived by experience in a constant state of Gratitude, it will seem as if one moment everything is going well and out of no where, you get "Tumble weeded." by Ego and Life is spiraling out of control. Don't fall for the trap of what your physical eyes report. Remember Who You are, a PERFECT creation of LOVE, and your perspective will be returned to Reality where life is appreciated for what it Really is, a precious gift, no matter how challenging your circumstance. Life situations are too often mistaken for Life itself which is where depression, resentment, anger, etc., comes in.
Remember the most Powerful tool you have innately within, LOVE, and Fear will instantly be a thing of the Past because Fear is of Ego and the Past. Let it ALL go. Stay awake. Stay Present in the Now. Be grateful for everything at all times because everything happens for only one reason and that is to learn from it! This will lessen your chances from getting "Tumble weeded." When it does happen, and it will, get back up, understand why it happened, which is always because you lost track of Reality then return to Truth. Spend not a moment more in what can never be undone, the Past, or worried about what you don't control anyway, the Future. Life is Now. Live it. Enjoy it. Experience it Now by just taking a few deep cleansing breaths and you will be reminded of ALL that you forgot; We Are One. Peace