y˘e (也) generally points to the similarity between two values that are in contrast in a given context.
我吃魚，他也吃魚 (w˘o ch¯ı y´u, t¯a y˘e ch¯ı y´u/ I ate fish, so did he.)
→ the doers, w˘o and t¯a, of the activity of fish eating are in contrast.
我吃魚，也吃肉 (w˘o ch¯ı y´u, y˘e ch¯ı r`ou/ I eat both fish and meat.)
→ the undergoers, fish and meat, are in contrast.
他昨天吃魚，今天也吃魚 (t¯a zu´oti¯an ch¯ı y´u, j¯ınti¯an y˘e ch¯ı y´u/ He ate fish yesterday, and he ate fish again today.)
→ the times, yesterday and today, are in contrast.
Reference: Chinese: a linguistic introduction by Chao Fen Sun