The sailor was now lost to them; and Susan's staunch, religious adherence to him as her husband, till her views had been disturbed by enlightenment, was demanded no more. (4.9)
Susan's loyalty to the sailor, Captain Newson, is not based on love but on a sense of religious duty.
She looked at him quite coolly, and saw how his forehead shone where the light caught it, and how nicely his hair was cut, and the sort of velvet-pile or down that was on the skin at the back of his neck, and how his cheek was so truly curved as to be part of a globe, and how clearly drawn were the lids and lashes which hid his bent eyes. (7.13)
This is Elizabeth-Jane's first good look at Farfrae. He's reading a newspaper and hardly notices that she's there, so she's able to check him out without his realizing it. Her eyes rove over his entire face and neck – it's a surprisingly intimate description, especially from the point of view of an innocent young girl.
He seemed to feel exactly as she felt about life and its surroundings – that they were a tragical, rather than a comical, thing; that though one could be gay on occasion, moments of gaiety were interludes, and no part of the actual drama. It was extraordinary how similar their views were. (8.33)
Elizabeth-Jane has never personally spoken with Farfrae – she's only heard him chatting with the townspeople. She has developed a crush on him from a distance and assumes they were made for each other. They think the same way about everything!
This young creature was staying at the boarding-house where I happened to have my lodging; and when I was pulled down she took upon herself to nurse me. From that she got to have a foolish liking for me. Heaven knows why, for I didn't encourage any such thing. But, being together in the same house, and her feelings warm, there arose a terrible scandal, which did me no harm, but was of course ruin to her. (12.23)
Love can ruin a girl's reputation, so watch out. Henchard isn't very delicate in his description of how Lucetta fell in love with him; he calls it a "foolish liking" that he didn't "encourage" at all.
Nobody would have conceived from his outward demeanour that there was no amatory fire or pulse of romance acting as stimulant to the bustle going on in his gaunt, great house; nothing but three large resolves: one to make amends to his neglected Susan, another to provide a comfortable home for Elizabeth-Jane under his paternal eye; and a third to castigate himself with the thorns which these restitutory acts brought in their train. (13.8)
Henchard isn't in love with Susan; he's only marrying her because he feels obligated to.
But Donald Farfrae admired her, too; and altogether the time was an exciting one; sex had never before asserted itself in her so strongly, for in former days she had perhaps been too impersonally human to be distinctively feminine. (15.5)
Once Elizabeth-Jane starts wearing fancier clothes, all the young men of Casterbridge (including Farfrae) start to admire her.
He was the kind of man to whom some human object for pouring out his heat upon – were it affective or were it choleric – was almost a necessity. The craving of his heart for the re-establishment of this tenderest human tie had been great during his wife's lifetime, and now he had submitted to its mastery without reluctance and without fear. (19.29)
Henchard needs human interaction as a vent for his passionate nature, whether it's someone to yell at ("choleric") or someone to be kind to ("affective").
But in the interval she – my poor friend – had seen a man she liked better than him. Now comes the point: Could she in honour dismiss the first? (24.57)
Lucetta makes up a story about "a friend" in order to ask Elizabeth-Jane what she should do. She has basically promised to marry Henchard, but that was a long time ago. Now she's met a man "she liked better" – Farfrae. What's a girl to do?
'I won't be a slave to the past – I'll love where I choose!' (25.24)
Lucetta asserts her freedom here. She won't "be a slave" to promises she made in the past; she'll choose her own path. This is a pretty radical thing for a woman to say back when it wasn't considered proper for a woman to show that she loved a man in any obvious way until after the man had already proposed.
Yet to Elizabeth-Jane it was plain as the tow-pump that Donald and Lucetta were incipient lovers. More than once, in spite of her care, Lucetta had been unable to restrain her glance from flitting across into Farfrae's eyes like a bird to its nest. (26.21)
Elizabeth-Jane can see right through Lucetta, in part because she's in love with Farfrae herself. Hardy is fond of using bird imagery, and this is one instance where the imagery isn't totally depressing: Lucetta's eyes are like birds, and Farfrae's are like a cozy little nest. When their eyes meet, it feels like home to Lucetta. Isn't that a sweet metaphor?