#affirmative language

Every time you'd say "but," change it to "and" and BE AMAZED by how it changes how what you say is taken.  "I really like you, but--" the listener will shut down, you may perceive that, and this may make you panic and rush through what you were going to say.

whereas

"I really like you, and--" the listener will not shut down, you will be forced to consider what you're about to say to make sure that you still convey what you mean to convey -- but in that moment your brain will have time to think a little bit more. 

Even if in both situations you go on to tell someone something they probably don't want to hear, you're more likely to convey it accurately and compassionately, and they're more likely to pick it up in a way that isn't harmful to them.