i could go into rehashing research that shows lifting weights makes you mentally sharper. or i could point out that you'll closer resemble the idealized media image of a man, which helps build your confidence around other men and women. i could also point out that your testosterone production increases, further developing your aggression and sex drive. and i certainly won't point out that lifting weights makes you much stronger, giving you the stamina and energy to lift heavy things and last longer in bed, making you a better lover.
and while all of the above are certainly valid reasons to lift weights and work out, the real reason you should work out, why you should lift weights, is that the women at the gym are likely to have benefitted the same pluses. the women at the gym are likely to be more confident, more outgoing, more aggressive, more energetic, better in bed, and closer resemble the idealized media image of a woman. especially if they lift weights, but sticking to the treadmill is just as good for them.
if you want better women, you have to change where you look. know what you want from a woman, figure out the likeliest place to find someone who matches that description, and go there to look for her.
but here's the kicker.
if you want someone whose truly compatible with you, you won't have to go anywhere special. you won't have to figure out where to meet girls. you just have to be active about pursuing your interests, your hobbies, your daily life. and whatever women you happen to meet in the course of doing so are likely to be your best bet.
the only question you have to answer then is, "am i attracted to them?" because you'll already be the man they are attracted to. so you have to find selfish reasons for doing what you do. reasons that benefit you even if other people turn their backs on you for doing so. reasons even for going to the gym.
you cannot live your life for others. only yourself.