SQL Server 2016 AlwaysON
You have a two-node setup and no witness, meaning there would be only two votes in the quorum. This is why you cannot use automated failover currently--there aren't enough votes, and as such, the remaining node cannot get quorum when the other node is offline.

There are two problems with this setup.



Split Brain

If a network outage occurs, the two nodes can no longer communicate, so without a third vote (file share or disk witness), neither node has a way of knowing which cluster resources the other node has online.  Accordingly, the resources will be taken offline to avoid a "split brain," which is a term used to describe a situation where multiple nodes have the same resource(s) online.



Quorum

In order for a cluster to be considered healthy and online, over 50% of votes must be cast.  In this scenario the moment one node goes offline, the cluster will go offline.  With only two votes available, each vote is worth 50%, so if one node is down or they cannot communicate, neither node can get more than 50%.



Enter the Witness

For the above reasons, you need to utilise a witness (fileshare or disk) to ensure that over 50% of votes are cast when one node goes down, and the cluster remains healthy and online. 

In your specific scenario you would need to utilise a file share witness or Azure witness, due to a lack of shared disks in the cluster.

If you are using Windows Server 2012 R2 and SQL 2012, you could also consider using Dynamic Witness rather than a witness.