In Silas Marner, we are told that "Marner's inward life has been a history and a metamorphosis." What has his life been filled with before he comes...

The quote that is mentioned in this question continues on, saying that his metamorphosis is often typical of someone who has 

[has] fled or been condemned to solitude.

This is very true of Silas Marner's life, and it is also true that his inner life had been a metamorphosis. 

The presence of Silas in Raveloe is the consequence of a terrible event that took place in his former town of Lantern Yard, where he had led a very full and happy life prior to the event. 

 Prior to moving to Raveloe, Silas’s life had been filled with

Movement, mental activity, and close fellowship.

The narrator goes on to say that Silas was once able to achieve all of these qualities because he had been influential in a “narrow religious sect” where he was highly thought of.

Surrounded by friendship and support, Silas excelled in this group, even becoming somewhat legendary after a time during which he fell into a deep trance during a prayer-meeting. This reaction was attributed to some particular spiritual gift or ability that is unique to Silas.

Among his friends there was William Dane, his closest one, who was also considered to be quite influential in the group. Aside from William, Silas had a girlfriend named Sarah, who was a young servant. He intended to marry Sarah once they saved up enough money.

While he enjoyed his time in Lantern Yard, Silas was respected, loved, and supported. He was not the strange and enigmatic, big-eyed loner that people saw in Raveloe. This altered persona, the Silas from Raveloe, is the product of the transformation, or metamorphosis, that he suffers as a result of the horrible betrayal that is orchestrated by William, who literally makes the whole congregation gang up against Silas.

William Dane was a fake friend, and first spread a rumor that Silas’s fits were more sinister than saintly in nature. To add insult to the injury, William also frames Silas and manages to set him up as a thief. Finally, William takes Sarah from Silas and marries her.

Imagine how Silas’s life must have changed as a result of this horrible betrayal. He was secure, happy, and strong in Lantern Yard, believing that his life was set. Now he would have to start over again in a new town, alone, and trying to make peace with what they did to him so unfairly.

The life of Silas in Lantern Yard ends in shame and heartbreak. This is a tough place to rise up from. This is what his life had previously been filled with and this is why his only choice was to transform his life altogether through the enforced metamorphosis from sociable to loner.