It is my firm opinion that almost everyone can benefit from training with another human. Why? Read on.
Do you train by yourself? If so, you are potentially missing out on a SIGNIFICANT amount of progress that you could be getting, for FREE. That thing you are missing out on is the training partner effect.
The case for training with a partner(s)
Most of us are in complete denial of just how much our own minds will bullshit us on a daily basis. Human beings are built to maximise comfort and minimise effort; it's a key instinct that has probably been vital in our successful evolution. While this instinct is useful for not starving to death in a Palaeolithic era caveman tribe, it's not great for adding 20kg to your squat.
Our desire to minimise energy expenditure encouraged us to invent fire and avoid freezing to death. in caveman times Now it's calling you home to your indoor heating, and away from the gym.
If you are someone who trains by themselves, then your mind is likely playing MASSIVE tricks on you, telling you that you're more tired than you really are. After all, it's been a tough week and you deserve a 'light day' [world's tiniest violin plays in the distance].
In reality, you're probably barely tapping into your work potential. This is where training partners come in.
Training with one or more people does a few really positive things for you and your training:
On those days where you feel sluggish and have zero motivation to train, it's a lot harder to justify ditching your gym session if it means explaining your lack of willpower to a training partner. They're waiting on you, depending on you. Don't let them down.
There's few things as damaging to your progress as training in an environment where you're the strongest person. This creates a false sense of superiority, a sort of 'big-fish-small-pond' syndrome of sorts.
However if you have a training partner who is at a similar level, you will likely feel motivated to match their achievements if they perform better than you (assuming you have some level of competitiveness).
This competitive environment is so important that it is the reason why many powerlifters train as part of a team. In fact, the highly successful powerlifting gym Westside Barbell advocates always having two kinds of training partners: one who is slightly weaker than you, and one who is slightly stronger than you. The theory is that the weaker partner keeps you motivated by threatening to overtake you if you start to slack off, while the stronger partner gives you someone to chase and motivates you to push hard.
The lifters of Westside Barbell train in groups.
As an aside, this is in my opinion one of the main reasons why CrossFit has become such a successful gym model. Group classes where everyone is competing is generally a lot more engaging for the average joe than working out by themselves in a commercial gym.
A training partner can give you instant feedback on how hard a set looked (something that most people will massively exaggerate in their head). Many times this has led to me PR-ing thanks to the good advice of a training partner.
The other critical type of feedback that a partner can give you is about your technique. Maybe they saw something that you couldn't feel happening, and correcting this could significantly improve your next set.
When the weight gets heavy, a good training partner will give you that extra peace of mind that you aren't going to get buried under a weight.
This is particularly important for the bench press, where a solid lift-off can be crucial to making the lift, and where the dangers of missing the rep with no spotter can be life threatening.
Training partners can hugely enhance your success as a lifter. Yes there are a few exceptions to this rule; people like Mike Tuscherer or Blaine Sumner, who are ridiculously strong despite training in solitude - but this is rare, and you probably aren't those guys.
So my advice is to find yourself someone who is at a similar level to you, and who is as committed to training as you, and start training like savages together.
The results will speak for themselves.