As a man's point of view, we, boys are losing interest to a girl if we are not having our time with them. Meaning, instead of doing something together with our girlfriend, we chose to spend our time doing something with our other friends and the worst is with other girls. These are just the common nature of man is. I just want to be honest with you but once your boyfriend is losing interest to you it means that he is attracted to some other girls or worst is he is already seeing or dating other girls. You will notice it by his own actions and on how he talked and how he spent his time with you. If he is always mad at you just because of little things and if he doesn't want to spend time with you, as if it seems like it is fine with him not seeing you around then there is something going on to your boyfriend.