Learning to be better involves paying attention to what you do, breaking it down and improving it. Because of this, some part of your focus (30% for instance) is on yourself, and you can only spare 70% on observing the situation.
In comparison, the button masher can keep 100% of his attention on you. That's why he is likely to win more for a while.
Of course once you improve to the point where you are even (your 70% equals his 100%) and slightly over, he suddenly doesn't have even a shadow of a chance. You will only keep improving, while he stays put. If he wants to chase after you, he'll have to do the same thing you did (to divert some of his focus on himself) but then he'll lose even more miserably for a good while. So he's not very likely to do it at this point. This is the real reason why button mashing sucks - if your playmate is a button masher, before long there is no point in playing against him at all.
About what Fire Daemon said (what if he isn't a button masher but just better than you?). Whether this is so is very easy to test. Just put 100% of your focus on winning, don't try to do anything fancy but button mash yourself! If the result of that test is that the score evens up, then he's winning due to the button mashing, which - like I said - is short lived. If the score doesn't even up, he's simply a better player than you and you need to improve to beat him.
In fighting games, the easiest way to beat a button masher is keeping your distance. Learn by heart a small handful of high-damage, relatively fast and safe attacks for different ranges. Poking low attacks that knock him down are gold! Pre-emptively control the distance by dashing away. (In the case of SC, also learn one good, fast, mid-range horizontal attack you can use to catch the opponent if he closes in by diagonal run.) From every distance, snipe with the best attack for that particular distance. The button masher, not planning his moves, is very unlikely to use a move that has enough range to get to you, while your move always gets him.