Why is friendship frowned upon by most parents?

I do not think many parents frown at friendships. Infact, I think most parents encourage it. Most parents frown at friendships when they notice their children are friends with people that have negative influence on them. Parents go all out to cut off such friends from their children.

Most especially at teenager stage, where kids like to explore and discover more, parents are always very watchful of the friends their children keep. Aside for this reason, I do not think parents frown against friendships.

I'm not sure you're right. Most parents want their children to make firm friends directly they start school proper. I suspect you're talking about friendships in your teens. Parents may seem to major on 'the right' friends, and 'nice' friends, but of course just because someone looks conventional and can turn on the neat and clean manners, doesn't make them nice people, as you well know.

It may be that parents get nervous when kids bring home friends that are 'different' fearing drugs and trouble, You have to be rather mature about this and think whether you actually really like and want to be like this friend, or whether they are just a means of looking cool while you are with them - or worse, that you fear problems if you don't continue in friendship with them.

If the latter, that's a warning sign, and if your parents are also voicing worries, perhaps they are right. Otherwise, make your own judgements. Do you really like this person? If you had a kid sister or brother, would you want them to behave like this person, be like them? It's all a case of thinking it over calmly.