I've killed many, many plants over the years. The large majority were unintentional. If I truly had a green thumb there wouldn't be so many vegetative ghosts in my garden. The deaths were caused primarily by my lack of information, lack of work, or lack of patience.
Even after months of study and tests to become a master gardener I still occasionally make mistakes and my plants suffer for it. To my credit, I've had many successes since I learned about what it takes to grow plants properly. Since becoming knowledgeable about gardening, the death rate in my garden has declined dramatically.
The basics growing plants are simple. Garden plants need soil, light, water, air, and nutrients. How strong they grow is determined by the quality of each of those components. Water and fertilizer in abundance won't help a plant in bad soil, just as they won't help a plant living in darkness. Perfect soil is useless without water, air, and sun. Successful gardening happens when the gardener is able to provide the correct balance.
Left to themselves, most garden plants would perish. Native plants in every region have adapted to survive without any intervention by man, but few plants in a typical garden are native. Your plants need your assistance. If you supply them with the aid they require, they'll reward you and themselves with sustained growth, flowers, and fruit. Pay too little attention or effort and they'll suffer.
Those are the first two components of successful gardening: knowledge and effort. You have complete control over gaining the information you need to garden in your area and spending the time to make it happen. Selecting the correct plant, putting it in the correct location, and giving it the correct attention happens because you find out about the specific needs and then make it happen.
That's all there is to it. Successful gardening is simple. Not easy, but simple. I'm still learning new things about gardening so that I can continue to have success with it. I spend time planting, fertilizing, weeding, harvesting, and propagating. I try new things. And I keep at it even when things don't turn out as planned. Gardening always offers new challenges. That's one of the reasons I love it.