I've always said that it's a terrible idea to make another person your source for motivation, specially a girl like someone you're dating because they

Can leave at any time, however, never have I felt so motivated as those times when I actually cared about a girl and wanted to be a better version of myself for them. Is it like some primitive mechanism of wanting to provide to get the woman or why is that?

Sometimes I'll joke around with friends and say I wish I was able to fall in love like I did when I was younger just so I could get that boost of motivation, also when you're heartbroken sonetikes you're really motivated