For lend/lease, the UK did pay back the US. The final payment was made in 2006.
I don't know if the USSR or China did (both of whom got lend-lease assistance, I believe).
There was a lot of bickering back then, as well. Bickering over where to attack, who is in charge of what, which regions were to fall under which country's oversight afterwards, etc.
Chuchill and Patton (and a lot of others) felt that the US and UK would be enemies of the Soviet Union and vice versa shortly after WWII (which turned out to be correct) and had ideas on what to do at the end of the WWII with that in mind. There were a lot of people in the west who were deeply uncomfortable with communism and Stalin's purges, wherein a couple million Soviets were arrested and put in labor camps or executed.
The USSR didn't start off as a US and UK ally in WWII. At the start of WWII, Germany and the USSR had an alliance that included a specification of how to divide up Europe between them. Both invaded Poland at the start of WWII in 1939, each taking their specified half. Shortly thereafter, the USSR took over Estonia, Latvia, and Lithuania and invaded Finland, while the Germans went for Scandanavia and France.
Only after Germany reneged upon the alliance and invaded the USSR in 1941 (Operation Barbarossa) did the USSR become a US and UK ally.
It's interesting to ponder what would have happened if the UK had declared war on the USSR like they did on Germany for invading Poland or if the Germans had not reneged on the initial German/USSR alliance.