The ShareThis widget tries to automatically pick the best data(image/descriptions,etc..)for sharing from the url you share , but sometimes our algorithm doesn't pick it up for some reason or another.
Sharethis facebook button implements a direct post, meaning user is redirected to the website. Because of this, Facebook is responsible for crawling information from the page. They do this via meta tags. If these tags do exist on the page, Facebook will display that information in the post.
Facebook uses properties “description” and “title” in the meta tags.
Wordpress has ways to put meta tags. Typically, this is done in the header.php template for the theme.
There are also wordpress plugins that setup these tags automatically. For ex: Yoast WordPress SEO