Why does he care one minute and then not the next?

I've been going out with my boyfriend for four years (on and off at points) and over the last 9 months or so we've been having arguments about everything but we decided that we were going to go travelling together this summer and he was so excited and we were planning it and then now he's just told me he's not sure whether we should go, and that he wasn't really that excited. He said he might not be able to get time off work (he works part time in a restauarant so that's a sh*t excuse!) and it's coming across like he really just doesn't care about me and us anymore otherwise he'd be more understanding to why I'm upset!
Why does he care one minute and then not the next?
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