There's a big possibility that Marco was treating Star the same way that Jackie was treating Marco.>If I really think about it, I've always kinda known that maybe Marco likes me... as more than a friend.>because I really valued him as a friend and didn't want to ruin our friendship, I kinda ignored the signs.
Basically, Jackie thought she didn't have a crush on Marco and had a crush on some other skater dude. So she thought if she acknowledged Marco's feelings, it would ruin their platonic friendship. Then, he actually confessed, and suddenly she wasn't sure whom she liked anymore. Turns out, after she thought about it for a while, she really liked Marco more than the skater dude.
Let's go ahead and do a quick substitution:
Basically, Marco thought he didn't have a crush on Star and had a crush on some other skater girl. So he thought if he acknowledged Star's feelings, it would ruin their platonic friendship. Then, she actually confessed, and suddenly he wasn't sure whom he liked anymore. Turns out, after he thought about it for a while, he really liked Star more than the skater girl.
This could potentially be some blatant foreshadowing.