Most Helpful Guy
Men are fickle creatures, sometimes. If a woman comes along that picks the right time to flirt, and his heart is not dedicated to you, it can seem to him like he has rediscovered happiness. His wandering heart may be a symptom of many things, including a relationship between you that has been growing stale, or even something like a fight that has left him feeling detached from you.
Or, it could be that he keeps some parts of his heart to himself, and this new woman is appealing to them. (Or... So he thinks. The truth is, he will not gain the satisfaction from her that he really desires.)