Blood clots are needed to help our body's repair an injury that has occurred to a blood vessel, but the issue is when our body becomes confused and tries to repair something that isn't damaged.
A blood clot in the heart usually occurs because a person's artery has had plaque built up inside and eventually, that plaque can break away. When this happens, the body mistakenly thinks it is damaged, and a blood clot is formed to repair it. Instead of dissolving, the blood clot in the heart attaches to the artery's opening, preventing the heart from receiving oxygen. The heart will begin to die without oxygen, and this causes a heart attack.