I think in the case of Cassandra she would be happier if no one drank from the Well. She doesn't approve of messing with strange old magic that you really don't understand. She doesn't like the idea of Morrigan drinking because she probably doesn't feel she entirely trusts her. However, she definitely doesn't want the Inquisitor to drink because of the inherent risk of allowing the only person who can seal rifts and is the leader of your organisation from potentially falling under the control of some unknown power. Hence, her disapproval no matter what you do.
As for Solas, it is pretty much along the same lines. He doesn't want Corypheus to get hold of the Well. He doesn't like the idea of Morrigan drinking from it. He doesn't want to drink himself but he doesn't want you to drink either, because of placing yourself under the control of the entity known as Mythal. The moment you enter the inner sanctum he realises she isn't entirely dead, although for obvious reasons he doesn't want to reveal too much. He might also be pulling a double bluff with Morrigan, knowing that by expressing disapproval she is going to be all the more determined to drink. After all he didn't warn her of the dangers, simply suggested she was too eager.
What was missing from Solas' dialogue was him specifically warning you off. He claims he did so back at Skyhold but of course he really didn't. The writer claims this was an oversight. Nevertheless the fact that he was himself so reluctant to do so, set alarm bells going with me. So like Cassandra, he really feels that drinking from the Well is a bad idea in normal circumstances but a necessary evil given the situation. This is why he disapproves no matter what you do.