We get this question almost on a daily basis. The problem is that there are millions of apps that use a notification icon. And there are probably thousands apps that uses a heart icon. And not all of them are dating icons. I would say majority of them are fitness, health, and self improvement apps. So unless it is something popular like Facebook or YouTube, most of these questions go unanswered.
Just ask the person what it is. Otherwise if this is your phone, then look in the notifications to find out what it is.