Inspirational quotes are exactly that, they come from inspired thinking. True inspiration comes from within. You are inspired when your higher self or your frontal lobe is directing you. When you are inspired, it feels right. Logically and emotionally you are convinced you are doing the right thing. It’s called your intuition, your inner guide.
Motivation is usually triggered by external events or situations. Unlike inspiration. Therefore while being motivated is really useful, and inspirational quotes can be motivational, it’s more of a short term fix.
Many inspirational quotes stem from universal wisdom. This equates well with spiritual growth. When we talk about spiritual growth, we mean we align with our higher self. Some people call that god or Jesus or many other religions. You don’t however have to be religious to believe in spirituality.
And finally, we come to love. Love is who we truly are, it is our intrinsic nature. Therefore many inspirational quotes come from love. Or they encourage us to love ourselves and others more. Many inspirational people are so because they approach their lives and vocations with love.