Why can't we form an alliance with Russia now that the Soviet Union no longer exists?

There is nothing preventing us from forming an alliance with Russia. There also was nothing that prevented us from forming an alliance with the Soviet Union. During World War II, we had a military alliance with the Soviet Union. However, after World War II ended, we had so many differences and disputes with the Soviet Union, many over the spread of communism, that we found it hard to make alliances with them.
What makes it difficult to form an alliance with Russia is that we seem to constantly be at odds with them. Just like with the Soviet Union, we have major differences of opinion with Russia on significant issues. For example, we don’t agree on how Syria should be handled. Russia has been a long-time ally of Syria while we view that government as a sponsor of terrorism. Russia would like to support the Syrian government while the United States would like to replace it with a different government. We also don’t agree with Russia’s policy of expansionism in areas bordering Russia. These differences make it harder to form an alliance with them since both sides lack trust in each other. However, it is possible to overcome these barriers in order to make mutually beneficial alliances.
https://foreignpolicy.com/2015/11/19/situation-report-can-the-u-s-and-russia-form-an-alliance-over-syria-the-french-way-of-war-syrian-hospitals-in-the-crosshairs-what-sinjar-has-hidden-u-s-bombers-to-australia-russian-artillery-i/

https://www.forbes.com/sites/realspin/2015/10/01/in-russias-careful-expansion-game-belarus-is-moscows-next-target/